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Objectives Quadriceps weakness persists after anterior cruciate ligament reconstruction. Muscle atrophy and activation failure may contribute. This study examined the roles of atrophy and activation failure in quadriceps weakness after anterior cruciate ligament reconstruction.

Laboratory-based experiment using a pretest/posttest design.To determine the effects of neuromuscular fatigue on quadriceps strength and activation and sagittal and frontal plane knee biomechanics during dynamic landing following anterior cruciate ligament reconstruction (ACLR).Impaired quadriceps central activation occurs post-ACLR, likely altering lower extremity biomechanics. Neuromuscular fatigue similarly reduces volitional muscle activation and impairs neuromuscular control. Upon return to full activity post-ACLR, individuals likely concurrently experience quadriceps central activation deficits and neuromuscular fatigue, though the effects of fatigue on muscle strength and activation and biomechanics post-ACLR are unknown.Seventeen individuals 7 to 10 months post-ACLR and 16 controls participated. Quadriceps strength and central activation ratio were recorded prefatigue and postfatigue, which was induced via sets of double-leg squats. Knee biomechanics were recorded during a dynamic landing activity prefatigue and postfatigue.Both groups demonstrated smaller knee flexion (initial contact, P = .017; peak, P = .004) and abduction (initial contact, P = .005; peak, P = .009) angles postfatigue. The ACLR group had smaller peak knee flexion angles (P.001) prefatigue and postfatigue than controls. Knee flexion moment was smaller in those post-ACLR than controls prefatigue (P.001), but not postfatigue (P = .103). Controls had smaller knee flexion moments postfatigue (P = .001). Knee abduction moment was smaller in both groups postfatigue (P = .003). All participants demonstrated significantly lower strength (P.001) and activation (P = .003) postfatigue.Impaired strength, central activation, and biomechanics were present postfatigue in both groups, suggesting that neuromuscular fatigue may increase noncontact ACL injury risk. However, these changes were not exaggerated in those post-ACLR, likely because they already demonstrated a stiff-legged landing strategy prefatigue.

Background:A reduced range of hip internal rotation is associated with increased peak anterior cruciate ligament (ACL) strain and risk for injury. It is unknown, however, whether limiting the available range of internal femoral rotation increases the susceptibility of the ACL to fatigue failure.Hypothesis:Risk of ACL failure is significantly greater in female knee specimens with a limited range of internal femoral rotation, smaller femoral-ACL attachment angle, and smaller tibial eminence volume during repeated in vitro simulated single-leg pivot landings.Study Design:Controlled laboratory study.Methods:A custom-built testing apparatus was used to simulate repeated single-leg pivot landings with a 4×-body weight impulsive load that induces knee compression, knee flexion, and internal tibial torque in 32 paired human knee specimens from 8 male and 8 female donors. These test loads were applied to each pair of specimens, in one knee with limited internal femoral rotation and in the contralateral knee with femoral rotation resisted by 2 springs to simulate the active hip rotator muscles’ resistance to stretch. The landings were repeated until ACL failure occurred or until a minimum of 100 trials were executed. The angle at which the ACL originates from the femur and the tibial eminence volume were measured on magnetic resonance images.Results:The final Cox regression model ( P = .024) revealed that range of internal femoral rotation and sex of donor were significant factors in determining risk of ACL fatigue failure. The specimens with limited range of internal femoral rotation had a failure risk 17.1 times higher than did the specimens with free rotation ( P = .016). The female knee specimens had a risk of ACL failure 26.9 times higher than the male specimens ( P = .055).Conclusion:Limiting the range of internal femoral rotation during repetitive pivot landings increases the risk of an ACL fatigue failure in comparison with free rotation in a cadaveric model.Clinical Relevance:Screening for restricted internal rotation at the hip in ACL injury prevention programs as well as in individuals with ACL injuries and/or reconstructions is warranted.

The use of autografts versus allografts for anterior cruciate ligament (ACL) reconstruction is controversial. The current popular options for ACL reconstruction are patellar tendon or hamstring autografts, yet advances in allograft technologies have made allogeneic grafts a favorable option for repair tissue. Despite this, the mismatched biomechanical properties and risk of osteoarthritis resulting from the current graft technologies have prompted the investigation of new tissue sources for ACL reconstruction. Previous work by our lab has demonstrated that tissue-engineered bone-ligament-bone (BLB) constructs generated from an allogeneic cell source develop structural and functional properties similar to those of native ACL and vascular and neural structures that exceed those of autologous patellar tendon grafts. In this study, we investigated the effectiveness of our tissue-engineered ligament constructs fabricated from autologous versus allogeneic cell sources. Our preliminary results demonstrate that 6 months postimplantation, our tissue-engineered auto- and allogeneic BLB grafts show similar histological and mechanical outcomes indicating that the autologous grafts are a viable option for ACL reconstruction. These data indicate that our tissue-engineered autologous ligament graft could be used in clinical situations where immune rejection and disease transmission may preclude allograft use.

Context: Physical activity may affect the concentrations of circulating endogenous hormones in female athletes. Understanding the relationship between athletic and physical activity and circulating female hormone concentrations is critical. Objective: To test the hypotheses that (1) the estradiol-progesterone profile of high school adolescent girls participating in training, conditioning, and competition would differ from that of physically inactive, age-matched adolescent girls throughout a 3-month period; and (2) athletic training and conditioning would alter body composition (muscle, bone), leading to an increasingly greater lean–body-mass to fat–body-mass ratio with accompanying hormonal changes. Design: Cohort study. Settings: Laboratory and participants' homes. Patients or Other Participants: A total of 106 adolescent girls, ages 14–18 years, who had experienced at least 3 menstrual cycles in their lifetime. Main Outcome Measure(s): Participants were prospectively monitored throughout a 13-week period, with weekly physical activity assessments and 15 urine samples for estrogen, luteinizing hormone, creatinine, and progesterone concentrations. Each girl underwent body-composition measurements before and after the study period. Results: Seventy-four of the 98 girls (76%) who completed the study classified themselves as athletes. Body mass index, body mass, and fat measures remained stable, and 17 teenagers had no complete menstrual cycle during the observation period. Mean concentrations of log(estrogen/creatinine) were slightly greater in nonathletes who had cycles of 35 days. Mean log(progesterone/creatinine) concentrations in nonathletes were less in the first half and greater in the second half of the cycle, but the differences were not statistically significant. Conclusions: A moderate level of athletic or physical activity did not influence urine concentrations of estrogen, progesterone, or luteinizing hormones. However, none of the participants achieved high levels of physical activity. A significant number (17%) of girls in both activity groups were amenorrheic during the 3-month study period.

Background: While body mass index (BMI), a modifiable parameter, and knee morphology, a nonmodifiable parameter, have been identified as risk factors for anterior cruciate ligament (ACL) rupture, the interaction between them remains unknown. An understanding of this interaction is important because greater compressive axial force (perhaps due to greater BMI) applied to a knee that is already at an increased risk because of its geometry, such as a steep lateral posterior tibial slope, could further increase the probability of ACL injury. Purpose: To quantify the relationship between BMI and select knee morphological parameters as potential risk factors for ACL injury. Study Design: Case-control study; Level of evidence, 3. Methods: Sagittal knee magnetic resonance imaging (MRI) files from 76 ACL-injured and 42 uninjured subjects were gathered from the University of Michigan Health System’s archive. The posterior tibial slope (PTS), middle cartilage slope (MCS), posterior meniscus height (PMH), and posterior meniscus bone angle (MBA) in the lateral compartment were measured using MRI. BMI was calculated from demographic data. The association between the knee structural factors, BMI, and ACL injury risk was explored using univariate and multivariate logistic regression. Results: PTS ( P = .043) and MCS ( P = .037) significantly predicted ACL injury risk. As PTS and MCS increased by 1°, odds of sustaining an ACL injury increased by 12% and 13%, respectively. The multivariate logistic regression analysis, which included PTS, BMI centered around the mean (cBMI), and their interaction, showed that this interaction predicted the odds of ACL rupture ( P = .050; odds ratio, 1.03). For every 1-unit increase in BMI from the average that is combined with a 1° increase in PTS, the odds of an ACL tear increased by 15%. Conclusion: An increase in BMI was associated with increased risk of ACL tear in the presence of increased lateral posterior tibial slope. Larger values of PTS or MCS were associated with an increased risk of ACL tear.

Background Although studies have shown increased rates of chondral injury with delayed surgical treatment of pediatric anterior cruciate ligament (ACL) injuries, it is unknown if this is related to a delay in diagnosis and appropriate activity restrictions. The purpose of this study was to determine if the time from injury to diagnosis, time from diagnosis to reconstruction, and preoperative activity level correlate with the degree of cartilage injury seen intraoperatively. Methods A retrospective review of skeletally immature patients who underwent ACL reconstruction was performed. Patients were stratified based on the time from injury to diagnosis (≤6 vs. >6 wk), diagnosis to surgery (≤6 vs. >6 wk), and injury to surgery (≤12 vs. >12 wk). Weight-bearing status, brace usage, athletic participation, and meniscus tears were characterized as binary variables. Articular cartilage injury was graded on a scale of 0 to 3. Differences between groups were analyzed using the χ test. Results In total, 91 subjects with mean age of 13 years (range, 9 to 16 y) were included. In total, 71% were diagnosed within 6 weeks of injury and 40% underwent surgery within 6 weeks of diagnosis. No differences were found in the presence of medial or lateral meniscus tears or the grade of articular cartilage damage when groups were analyzed by time from injury to diagnosis, diagnosis to surgery, and injury to surgery (P>0.05 in all cases). When stratified by weight-bearing status, brace status, and athletic activity, there were no significant differences between groups for meniscal tears or cartilage injury in any compartment (P>0.05). Conclusions Diagnosis of ACL rupture within 6 weeks of injury and surgical reconstruction within 6 weeks of diagnosis or 12 weeks of injury do not appear to affect the rate of cartilage injury in skeletally immature patients. Weight-bearing status, brace use, and participation in athletic activities between the time of injury and diagnosis also did not impact the rate of intra-articular injury following ACL tear. Level of evidence Level IV.

PURPOSE: Evidence suggests that femoroacetabular impingement (FAI) in athletes may increase the risk of anterior cruciate ligament (ACL) injury. This study correlates ACL injury with hip range of motion in a consecutive series of elite, contact athletes and tests the hypothesis that a restriction in the available hip axial rotation in a dynamic in silico model of a simulated pivot landing would increase ACL strain and the risk of ACL rupture. METHODS: Three hundred twenty-four football athletes attending the 2012 NFL National Invitational Camp were examined. Hip range of internal rotation was measured and correlated with a history of ACL injury and surgical repair. An in silico biomechanical model was used to study the effect of FAI on the peak relative ACL strain developed during a simulated pivot landing. RESULTS: The in vivo results demonstrated that a reduction in internal rotation of the left hip was associated with a statistically significant increased odds of ACL injury in the ipsilateral or contralateral knee (OR = 0.95, p = 0.0001 and p < 0.0001, respectively). A post-estimation calculation of odds ratio for ACL injury based on deficiency in hip internal rotation demonstrated that a 30-degree reduction in left hip internal rotation was associated with 4.06 and 5.29 times greater odds of ACL injury in the ipsilateral and contralateral limbs, respectively. The in silico model demonstrated that FAI systematically increased the peak ACL strain predicted during the pivot landing. CONCLUSION: FAI may be associated with ACL injury because of the increased resistance to femoral internal axial rotation during a dynamic maneuver such as a pivot landing. This insight may lead to better interventions to prevent ACL injury and improved understanding of ACL reconstruction failure. LEVEL OF EVIDENCE: Cohort study, level IV.

All-arthroscopic tibial inlay double-bundle (DB) posterior cruciate ligament (PCL) reconstruction avoids an open dissection and the "killer turn" while maintaining the advantage of an anatomic graft. However, clinical data on the viability of this surgical technique in multiligamentous knee injuries are lacking.At greater than 2 years of followup, we evaluated (1) validated outcomes scores; (2) range of motion; and (3) side-to-side stability on PCL stress radiographs of a small group of patients who underwent all-arthroscopic tibial inlay DB PCL reconstruction in multiligamentous knee injuries, either shortly after injury or late.All patients sustaining an operative multiligamentous knee injury between August 2007 and March 2009 underwent PCL reconstruction with the all-arthroscopic tibial inlay DB PCL reconstruction. Twelve patients sustained such injuries and were reconstructed during the study period and all 12 returned for followup with a minimum of 2 years (mean 3 ± 0.8 years). There were nine males and three females, with a mean age of 30 years; four patients had a subacute reconstruction (≥ 3 weeks, but3 months), and eight patients had chronic reconstructions (3 months). Mean time from injury to PCL reconstruction was 7 ± 12 months. Demographics, ROM, outcome scores (Lysholm and International Knee Documentation Committee [IKDC] scores), and PCL stress views were obtained.At final followup, mean Lysholm and IKDC subjective scores were 79 ± 16 and 72 ± 19, respectively. IKDC objective scores included eight nearly normal knees, three abnormal knees, and one severely abnormal knee. Mean flexion and extension losses compared with the contralateral were 10 ± 9 and 1 ± 2, respectively. Mean ± SD final side-to-side difference on PCL stress radiographs was 5 ± 3 mm.The clinical and radiographic results of the all-arthroscopic tibial inlay DB PCL reconstruction appear comparable to the same technique in isolated PCL injuries and, based on similar published case series, comparable to results of multiligamentous knee reconstructions using other PCL reconstruction techniques.Level IV, therapeutic study. See the Instructions for Authors for a complete description of levels of evidence.

The purpose of this study was to determine the effect of concomitant meniscal surgery on the recovery of quadriceps activation and strength at a time when individuals return to sport following anterior cruciate ligament (ACL) reconstruction. Forty-six individuals that were cleared for participation following ACL reconstruction were invited to participate in this study. Participants were placed into groups according to surgical reports (ACL-only, n = 24; meniscal repair, n = 12; meniscectomy, n = 10). Quadriceps strength was quantified using isokinetic and isometric measures. Isokinetic strength was collected at 60°/s in concentric mode. Isometric strength was collected at 90° of knee flexion. Quadriceps activation was assessed using the burst superimposition technique and quantified via the central activation ratio. One-way ANOVAs were utilized to detect whether differences existed in quadriceps activation and strength between groups. Where appropriate, post hoc Bonferroni multiple comparison procedures were used. Quadriceps activation (P = n.s.) and strength (isokinetic: P = n.s.; isometric: P = n.s.) were not different between groups. Concomitant meniscectomy or meniscal repair did not affect the recovery of quadriceps activation and strength at a time when individuals return to sport following ACL reconstruction. Though group differences in quadriceps function were not detected, all participants demonstrated levels of quadriceps activation failure that are below healthy individuals at a time when they were returned to sport. Given that persistent quadriceps activation failure is detrimental to knee function, rehabilitation protocols that target quadriceps activation failure should be developed and employed post-reconstruction. III.

Fibular hemimelia is a rare congenital malformation that is commonly associated with other lower limb abnormalities. This is a unique case of a bicruciate ligament, anterior cru ciate ligament/posterior cruciate ligament (ACL/PCL) deficiency in a 6-year-old female with fibular hemimelia in which we describe an ACL reconstruction using autograft–allograft hybrid technique. This case focuses on the technical aspects of an ACL reconstruction using a physeal-sparing technique with a hybrid ACL graft in a pediatric patient with fibular hemimelia. When evaluating patients with fibular hemimelia, it is important to consider implications of treatment in a stepwise manner as this condition commonly presents with other abnormalities that will most likely require multiple procedures, including limb lengthening.

ACL injury prevention programs often involve strengthening the knee muscles. We posit that an unrecognized benefit of such training is the associated increase in the tensile stiffness of the hypertrophied muscle. We tested the hypothesis that an increased quadriceps tensile stiffness would reduce peak anteromedial bundle (AM-)ACL relative strain in female knees. Twelve female cadaver knees were subjected to compound impulsive two-times body weight loads in compression, flexion, and internal tibial torque beginning at 15° flexion. Knees were equipped with modifiable custom springs to represent the nonlinear rapid stretch behavior of a normal and increased stiffness female quadriceps (i.e., 33% greater stiffness). Peak AM-ACL relative strain was measured using an in situ transducer while muscle forces and tibiofemoral kinematics and kinetics were recorded. A 3D ADAMS™ dynamic biomechanical knee model was used in silico to interpret the experimental results which were analyzed using a repeated-measures Wilcoxon test. Female knees exhibited a 16% reduction in peak AM-ACL relative strain and 21% reduction in change in flexion when quadriceps tensile stiffness was increased by 33% (mean (SD) difference: 0.97% (0.65%), p = 0.003). We conclude that increased quadriceps tensile stiffness reduces peak ACL strain during a controlled study simulating a pivot landing.

Anterior cruciate ligament (ACL) ruptures rank among the most prevalent and costly sports-related injuries. Current tendon grafts used for ACL reconstruction are limited by suboptimal biomechanical properties. We have addressed these issues by engineering multiphasic bone-ligament-bone (BLB) constructs that develop structural and mechanical properties similar to native ACL. The purpose of this study was to examine the acute remodeling process that occurs as the BLB grafts advance toward the adult ligament phenotype in vivo. Thus, we implanted BLB constructs fabricated from male cells into female host sheep and allowed 3, 7, 14, or 28 days (n = 4 at each time point) for recovery. To address whether or not graft-derived cells were even necessary, a subset of BLB constructs (n = 3) were acellularized, implanted, and allowed 28 days for recovery. At each recovery time point, the following histological analyses were performed: picrosirius red staining to assess collagen alignment and immunohistochemistry to assess both graft development and host immune response. Polymerase chain reaction (PCR) analysis, performed on every explanted BLB, was used to detect the presence of graft-derived male cells remaining in the constructs and/or migration into surrounding host tissue. The analysis of the PCR and histology samples revealed a rapid migration of host-derived macrophages and neutrophils into the graft at 3 days, followed by increased collagen density and alignment, vascularization, innervation, and near complete repopulation of the graft with host cells within 28 days. This study provides a greater understanding of the processes of ligament regeneration in our BLB constructs as they remodel toward the adult ligament phenotype.

Background Most ruptures of the native anterior cruciate ligament (ACL) and ACL graft occur at, or near, the femoral enthesis, with the posterolateral fibers of the native ligament being especially vulnerable during pivot landings. Characterizing the anatomy of the ACL femoral enthesis may help us explain injury patterns which, in turn, could help guide injury prevention efforts. It may also lead to improved anatomic reconstruction techniques given that the goal of such techniques is to replicate the knee’s normal anatomy. Hence, the aim of this study was to investigate the microscopic anatomy of the ACL femoral enthesis and determine whether regional differences exist. Methods Fifteen human ACL femoral entheses were histochemically processed and sectioned along the longitudinal axis of the ACL at 20, 40, 60, and 80 % of the width of the enthesis. Four thick sections (100 μm) per enthesis were prepared, stained, and digitized. From these sections, regional variations in the quantity of calcified and uncalcified fibrocartilage, the angle at which the ligament originates from the bone, and the shape profile of the tidemark were quantified. Results At least 33 % more calcified fibrocartilage and 143 % more uncalcified fibrocartilage were found in the antero-inferior region, which corresponds to the inferior margin of the origin of the anteromedial ACL fibers, than all other regions (Ps

Objectives: Intervention strategies to prevent ACL injury rely on increasing knowledge of risk factors. While several modifiable and non-modifiable risk factors for ACL rupture have been identified, the interaction between them remains unknown. The aim of this study was to quantify the relationship between BMI and several knee geometries as potential risk factors for ACL injury. We hypothesized that an increased BMI in the presence of an increased posterior tibial slope or middle cartilage slope would increase risk of ACL injury. We also hypothesized that an increased BMI in the presence of a decreased posterior meniscal height or meniscal bone angle would result in an increased risk of ACL injury. Methods: Sagittal knee MRI files from 76 ACL-injured and 42 non-injured subjects were gathered from the institution’s archive. The PTS, MCS, PMH, and MBA were measured using the circle method and compared with BMI from the subject demographic. Data were analyzed using univariate and multivariate logistical regression. Figure 1 details measurements made for each knee geometry. Results: Univariate analysis of PTS showed increases in PTS significantly increase the odds of ACL tear (p = 0.043, OR =1.12). Univariate analysis of MCS showed increases of MCS significantly increase the odds of ACL tear (p = 0.037, OR = 1.12). Multivariate analysis of PTS and BMI centered around the mean (PTS*cBMI) showed increases of PTS in combination with increases in cBMI significantly increases the odds of ACL rupture (p value = .050, OR = 1.03). Table 1 shows predicted increases in ACL injury risk for combinations of increases in PTS and BMI. Conclusion: An increase in BMI will increase the risk of ACL tear when an increase in lateral posterior tibial slope is present. An increase in lateral posterior tibial slope or lateral middle cartilage slope increases the risk of an ACL tear.

I think most of us have fond memories of an outdoor venture that we did as kids that send chills down our spine as adults. It may have gotten us into trouble with parents or teachers and resulted in warranted disciplinary action with promises to never do that again, or it may have resulted in injuries that taught us valuable lessons after a trip to the emergency room. Hopefully, none ended in tragedy leaving scars for life, such as a youngster that dives into a shallow lake or river resulting in a cervical spine fracture and resulting quadriplegia, or worse. Life is full of inviting open doors that lead to excitement, intrigue, or adventure. Unfortunately, most kids do not recognize the potential risks and dangers because life’s experiences thus far have been limited and children have not learned those difficult life lessons yet. As we grow older, it is the collective effect of those lessons and/or bad memories—our own, that of our friends, and sometimes from our environment—that sway our choices for fun and adventure. For myself, white water rafting has always been fascinating and a lifelong desirable activity, having rocketed down several rivers while getting tossed about like a toy. The “shotgun” on the Salmon River in Idaho was my favorite! The beautiful surrounding landscape was always a welcomed feature. Regrettably, but fortunately, I have grown more cautious since those early trips on the white water. Looking back, there was one day on a family vacation in Costa Rica that I got talked into some rafting by my college-aged children. I’m pretty surprised that I did not carefully consider some of the dangers of the river with its hidden obstacles submerged below the surface that could crush body parts and threaten livelihood. While I had a lot of previous experience with fairly calm trips down rivers interspersed with rapids, I had not been subjected to hours of roller coaster–type white water punctuated by only brief 20- to 30-second rests between rapids. What a ride that was! I knew we were in trouble when my wife managed to stay in the raft for less than 2 minutes from the start of the trip. I wasn’t much better! I got a pretty good view of the depths of the river by getting thrown out and briefly trapped underneath the raft as we raced through raging rapids. The submerged boulders that I saw jetting by me while I was caught below the surface were impressive but quite scary. Questions of what level of trauma care was available in the villages along the river streamed through my mind as I struggled to get back into the raft. Those recurring questions strengthened my resolve to stay in the raft for the rest of the trip down the river. Needless to say, it was an educational experience. One that I had not expected! If I recognized some of the potential dangers before agreeing to the challenges of my kids, I probably would have rationalized what a problem a concussion or a fractured tibia would have been and where the medical care might be delivered. To be honest, I probably would have made other activity choices that day had I fully evaluated the risks and potential consequences. Clearly, we’re never too old to learn from our mistakes and short sightedness! For those of us who love the outdoors in this beautiful world in which we live, there is plenty to experience and enjoy as long as we are adequately prepared for what we may encounter along the way. For outdoor enthusiasts of all types, avoiding serious medical problems and injuries is the key to lifelong enjoyment of these opportunities. To help us stay out of trouble, there are several feature articles in this issue of Sports Health that should be of interest to outdoor men and women. “Athletes at High Altitude” is a concise overview of all of the problems that can be encountered not only by athletes but physically active people as they bike, jog, hike, or hunt above 2500 meters.8 While the title says “high altitude,” 2500 meters really isn’t that uncommon for a lot of activities in many regions of our mountainous states. As the oxygen concentration and barometric pressure decreases with altitude, physiologic changes occur in our bodies. Knowing which ones are indicators of serious consequence is of utmost importance. Slow ascent is the guideline but many people disregard safety guidelines. Once above 2500 meters, altitude should be increased at the rate of only 600 to 1200 meters per 24 hours.7 Adequate rest and recovery is also essential. Physically demanding activities depend on good restful sleep, but sleep patterns are often altered at high altitude.2,3 Without restful sleep, irritability and poor decision making can result in making many mountain adventures more difficult and possibly risky. While we usually associate children with sleep issues, they may be better suited than adults for altitude endeavors.8 Unfortunately, pregnant women may be the ones most at risk.4 Anyone who has spent some time in the mountains knows how quickly weather conditions can change as you gain altitude. Many a backpacker has gotten into trouble starting off on a sunny warm day in light clothing and being ill prepared for the drastic drop in temperature as winds increase and cloud cover produces rain, sleet, or snow. Adequate clothing is essential, including moisture-wicking layers at the skin surface. There is good justification for the adage that “cotton kills.” Those trips can turn tragic rapidly. Therefore, being well aware of the essentials in “Exercise in the Cold”5 is a must to survive and enjoy the trip when weather takes a turn for the worse. Hypothermia can be quite dangerous when it affects the nervous system, and subsequently, our decision-making skills. Clear thinking will often be the determining factor when faced with hazardous weather conditions. At the other extreme of weather conditions, dehydration and exertional heat stroke associated with endurance exercise and physical pursuits in hot weather can be just as dangerous. Multisystem illness is likely when core body temperature exceeds 104°F.1 Many endurance events occur in the summer heat and demand acclimatization to be safe. The pitfalls and guidelines for exercising in the heat are well described by Asplund and O’Connor1 in this issue. Last in this issue’s features on outdoor activities is an excellent review on the otologic hazards of scuba diving.6 More than once I’ve been tempted to take the 4-hour course at one of several local venues to get certified to dive. Many friends have described the inviting features of the deep, especially the fish life. After reading the review on the otologic hazards of scuba diving, I wonder how well the swimming pool courses prepare divers for the hazards. I think I’d now opt for a more extensive certification program if I were going to dive or just stick with snorkeling. Irreparable hearing loss wasn’t something that I’d associated previously with scuba diving. I hope all of you who love the outdoors as I do have time to enjoy these 4 feature articles on altitude, cold, heat, and diving. I found them very educational. I sincerely hope that these features will not dissuade anyone from enjoying our magnificent outdoor world but rather encourage a little more caution and preparation for the unexpected. I do hope that these features help us protect children and all those entrusted to our care on these adventures. Hopefully, these will help avoid unnecessary injury, illness, accidents, or tragedies. For me, the next time I depart on my annual bear hunting/fishing adventure in the wilderness of Manitoba, I will recheck my first aid kit and make sure that our guide’s satellite phone is in good working order. Safe travels!

Purpose The primary purpose was to compare ipsilateral hip internal rotation (IR) in male and female athletes with or without an anterior cruciate ligament (ACL) tear. A secondary purpose was to compare radiographic markers of femoroacetabular impingement (FAI) in patients with or without an ACL tear. Methods In this prospective case-control study, based on a power analysis, a convenience sample of 25 ACL-injured and 25 control patients matched by age and gender were examined over 14 months. The ACL injury group included preoperative patients 12-40 years old with an ACL rupture within the previous 3 months with no prior lower extremity injuries, ligamentous laxity, or arthralgias. Controls included patients presenting with an upper extremity complaint with no history of knee injury. In the outpatient clinic, hip axial rotation range of motion was measured with a goniometer on physical examination and hip radiographs were evaluated for morphologic variations consistent with FAI. Univariate analysis of variance was used to examine differences between groups. Results Each group had 13 males and 12 females, average ages of 22.8 ± 7.2 years (ACL group) versus 24.5 ± 7.9 years (controls; P = .439). The average sum of hip rotation (internal plus external) in patients with an ACL tear was 60.3 ± 12.4° compared with 72.6 ± 17.2° in controls ( P = .006). ACL-injured patients had decreased hip IR compared with controls, with respective mean measurements of 23.4 ± 7.6° versus 30.4 ± 10.4° ( P = .009). For every 10° increase in hip IR, the odds of having an ACL tear decreased by a factor of 0.419 ( P = .015). Conclusions Risk of ACL injury is associated with restricted hip IR, and as hip IR increases, the odds of having an ACL tear decreases. In addition, ACL injury is associated with FAI in a generalized population of male and female athletes, although causality cannot be determined and most ACL-injured patients do not exhibit hip complaints. Level of Evidence Level II, prognostic, prospective cohort study.

Background: Since lateral tibial slope (LTS) affects the amount of anterior tibial translation and anterior cruciate ligament (ACL) strain during a dynamic maneuver, accurate measurements of LTS may be beneficial in screening people at a higher risk for ACL injury. Methods for measuring LTS on magnetic resonance imaging (MRI) scans of the proximal tibia include the midpoint and circle methods. No current studies have validated different LTS measurement methods using a proximal tibia MRI scan. Hypothesis: We tested the null hypotheses that (1) LTS measurements were independent of the length of tibia imaged using the midpoint method, and (2) LTS measurements calculated from different methods (midpoint, circle, and full tibia) would not differ significantly. Study Design: Controlled laboratory study. Methods: Blinded observers measured LTS from 3-tesla, 3-dimensional MRIs from 40 size-matched donors according to 1 circle method and 3 midpoint methods. Outcomes were then compared with the full-tibial anatomic axis (line connecting the center of 2 circles fit within the proximal and distal tibia) in 11 donors. Bonferroni-corrected paired t tests (significance, P < .005) were used to compare the 5 methods. Results: The circle and full-tibia methods had the lowest interobserver and intraobserver variability, whereas the midpoint method with 10-cm tibia was the most variable. The midpoint method with 10-cm and 15-cm proximal tibia closely resembled LTS measurements with the full-tibial anatomic axis. The circle method, although repeatable, provided smaller numerical LTS measurements than the full-tibia and midpoint methods. Conclusion: Although LTS measurements using the midpoint method can resemble measurements made using the full tibia, the reliability of the midpoint method depends on the length of proximal tibia used. The circle method may be the preferred method for future studies since it was the most repeatable method and is independent of proximal tibial length. Clinical Relevance: LTS measurements vary depending on the method used.

Synopsis Knee injuries involving multiple ligaments, which are often associated with tibiofemoral joint dislocations, are complex injuries that are challenging to evaluate and treat. The clinician must have a thorough understanding of knee anatomy and be capable of a detailed physical examination to determine the extent and pattern of injury. Accurate interpretation of imaging studies is crucial but cannot replace the physical examination. The magnitude of deforming forces and amount of translation and rotation of the tibia in relation to the femur determine the risk for neurovascular injury. Most patients with multiligament knee injuries undergo surgical management; however, some patients may be best served by a nonoperative approach. Successful rehabilitation consists of achieving good range of motion and strength, as well as normal gait mechanics, while respecting the stages of healing. Level of Evidence Therapy, level 5. J Orthop Sports Phys Ther 2012;42(3):234–242. doi:10.2519/jospt.2012.3678

The anterior cruciate ligament (ACL), a major stabilizer of the knee, is commonly injured. Because of its intrinsic poor healing ability, a torn ACL is usually reconstructed by a graft. We developed a multi-phasic, or bone-ligament-bone, tissue-engineered construct for ACL grafts using bone marrow stromal cells and sheep as a model system. After 6 months in vivo, the constructs increased in cross section and exhibited a well-organized microstructure, native bone integration, a functional enthesis, vascularization, innervation, increased collagen content, and structural alignment. The constructs increased in stiffness to 52% of the tangent modulus and 95% of the geometric stiffness of native ACL. The viscoelastic response of the explants was virtually indistinguishable from that of adult ACL. These results suggest that our constructs after implantation can obtain physiologically relevant structural and functional characteristics comparable to those of adult ACL. They present a viable option for ACL replacement.

Anterior cruciate ligament (ACL) injuries most frequently occur under the large loads associated with a unipedal jump landing involving a cutting or pivoting maneuver. We tested the hypotheses that internal tibial torque would increase the anteromedial (AM) bundle ACL relative strain and strain rate more than would the corresponding external tibial torque under the large impulsive loads associated with such landing maneuvers. Twelve cadaveric female knees [mean (SD) age: 65.0 (10.5) years] were tested. Pretensioned quadriceps, hamstring, and gastrocnemius muscle-tendon unit forces maintained an initial knee flexion angle of 15°. A compound impulsive test load (compression, flexion moment, and internal or external tibial torque) was applied to the distal tibia while recording the 3D knee loads and tibofemoral kinematics. AM-ACL relative strain was measured using a 3 mm DVRT. In this repeated measures experiment, the Wilcoxon signed-rank test was used to test the null hypotheses with p

Background: Gender differences exist in anterior cruciate ligament (ACL) cross-sectional area and lateral tibial slope. Biomechanical principles suggest that the direction of these gender differences should induce larger peak ACL strains in females under dynamic loading. Hypothesis: Peak ACL relative strain during a simulated pivot landing is significantly greater in female ACLs than male ACLs. Study Design: Controlled laboratory study. Methods: Twenty cadaveric knees from height- and weight-matched male and female cadavers were subjected to impulsive 3-dimensional test loads of 2 times body weight in compression, flexion, and internal tibial torque starting at 15° of flexion. Load cells measured the 3-dimensional forces and moments applied to the knee, and forces in the pretensioned quadriceps, hamstring, and gastrocnemius muscle equivalents. A novel, gender-specific, nonlinear spring simulated short-range and longer range quadriceps muscle tensile stiffness. Peak relative strain in the anteromedial bundle of the ACL (AM-ACL) was measured using a differential variable reluctance transducer, while ACL cross-sectional area and lateral tibial slope were measured using magnetic resonance imaging. A repeated-measures Mann-Whitney signed-rank test was used to test the hypothesis. Results: Female knees exhibited 95% greater peak AM-ACL relative strain than male knees (6.37% [2.53%] vs 3.26% [1.89%]; P = .004). Anterior cruciate ligament cross-sectional area and lateral tibial slope were significant predictors of peak AM-ACL relative strain ( R2 = .59; P = .001). Conclusion: Peak AM-ACL relative strain was significantly greater in female than male knees from donors of the same height and weight. This gender difference is attributed to a smaller female ACL cross-sectional area and a greater lateral tibial slope. Clinical Relevance: Since female ACLs are systematically exposed to greater strain than their male counterparts, training and injury prevention programs should take this fact into consideration.

Background: Knee joint morphology contributions to anterior cruciate ligament (ACL) loading are rarely considered in the injury prevention model. This may be problematic as the knee mechanical response may be influenced by these underlying morphological factors. The goal of the present study was to explore the relationship between posterior tibial slope (which has been recently postulated to influence knee and ACL loading), impact-induced anterior tibial acceleration, and resultant ACL strain during a simulated single-leg landing. Methods: Eleven lower limb cadaveric specimens from female donors who had had a mean age (and standard deviation) of 65 ± 10.5 years at the time of death were mounted in a testing apparatus to simulate single-limb landings in the presence of pre-impact knee muscle forces. After preconditioning, specimens underwent five impact trials (mean impact force, 1297.9 ± 210.6 N) while synchronous three-dimensional joint kinetics, kinematics, and relative anteromedial bundle strain data were recorded. Mean peak tibial acceleration and anteromedial bundle strain were quantified over the first 200 ms after impact. These values, along with radiographically defined posterior tibial slope measurements, were submitted to individual and stepwise linear regression analyses. Results: The mean peak anteromedial bundle strain (3.35% ± 1.71%) was significantly correlated (r = 0.79; p = 0.004; β = 0.791) with anterior tibial acceleration (8.31 ± 2.77 m/s-2), with the times to respective peaks (66 ± 7 ms and 66 ± 4 ms) also being significantly correlated (r = 0.82; p = 0.001; β = 0.818). Posterior tibial slope (mean, 7.6° ± 2.1°) was significantly correlated with both peak anterior tibial acceleration (r = 0.75; p = 0.004; β = 0.786) and peak anteromedial bundle strain (r = 0.76; p = 0.007; β = 0.759). Conclusions: Impact-induced ACL strain is directly proportional to anterior tibial acceleration, with this relationship being moderately dependent on the posterior slope of the tibial plateau. Clinical Relevance: Anterior tibial acceleration is associated with anteromedial bundle strain during simulated landings. The magnitude of the impact-induced accelerations governing the strain response is additionally correlated with the posterior slope of the tibial plateau. Additional exploration of the effect of other knee morphological variables on ACL strain during simulated high-risk landings appears warranted.

BACKGROUND The purpose of this study was to evaluate the results of arthroscopic repair of the meniscus in children and young athletes. METHODS Arthroscopic meniscus repairs performed on 49 knees in 31 male and 14 female patients

Treating mulitligament knee dislocations can be difficult. Initial treatment should consist of immediate reduction and care for limb-threatening injuries. Physical examination, x-rays, and magnetic resonance imaging are often necessary to clearly define the injury pattern. Occasionally, a computed tomography scan is beneficial to better characterize fractures. Timing for definitive treatment of the multiligament knee injury depends on associated injuries at the time of injury, the integrity of the soft tissues, the vascular status of the limb, and the condition of the knee at presentation. In general, early surgery is indicated in patients with associated fractures, vascular compromise, and patellar or quadriceps tendon ruptures. We also tend to favor acute repair of lateral-sided injuries with or without graft augmentation. Medial collateral ligament (MCL) injuries can be treated with immobilization unless the MCL is displaced or if there is associated meniscal extrusion. The condition of the knee largely determines whether the cruciate ligaments should be acutely reconstructed or reconstructed in a staged procedure. We treat complete posterior cruciate ligament injuries with a double-bundle arthroscopic inlay technique. The decision to use allograft or autograft depends on the severity of the knee injury, the demands that will be placed on the knee, and patient wishes. The return to a high level of sports participation after this injury is challenging and usually takes at least 9 to 12 months.

Exercise across the lifespan is an important maintenance program for good health. Particularly in the American population, exercise is essential to prevent such conditions as obesity, diabetes, and cardiac problems, which often result in premature death. However, those who exercise by participating in sports from a young age can suffer debilitating injuries that may curtail their ability to exercise throughout their lives, thus hampering their efforts to stay healthy and avoid disease. EPIDEMIOLOGY

Purpose The purpose of this study was to determine whether dynamic ultrasound (US) would be able to differentiate posterolateral corner knee injuries that would require surgical intervention. Methods This is a single-group cohort study. A knee sonogram was obtained in patients referred to us with suspected posterolateral knee injury. In addition to static US imaging, a dynamic US stress test was performed by placement of maximum varus stress on the knee at 30° of flexion. The tibiofemoral separation was then measured with US. Results from US and surgery were then compared. Results Sixteen patients had US and underwent surgery, and twelve patients had surgical findings requiring surgical intervention to the posterolateral knee structures. With regard to static US images, the sensitivity, specificity, negative predictive value, positive predictive value, and accuracy were 92%, 75%, 92%, 75%, and 88%, respectively, for the lateral collateral ligament; 33%, 100%, 100%, 33%, and 50%, respectively, for the popliteus; and 67%, 75%, 67%, 75%, and 69%, respectively, for the popliteofibular ligament. The dynamic US stress test showing 10.5 mm of lateral joint space width or more during varus stress showed a sensitivity of 83% and specificity of 100% for injury to the lateral collateral ligament and posterolateral corner structures, with a positive predictive value of 100%, negative predictive value of 75%, and accuracy of 88%. Conclusions A positive dynamic US stress test (≥10.5 mm) positively predicted the need for posterolateral knee surgery in 100% of patients in this study who required posterolateral corner repair or reconstruction. Level of Evidence Level I, testing of previously developed diagnostic criteria in series of consecutive patients with universally applied gold standard.

Purpose The purpose of this study was to investigate complications of the medial opening wedge high tibial osteotomy (HTO) procedure at our institution. Methods All cases of medial opening wedge HTO performed between 2001 and 2004 at our institution were identified. Medical records, operative reports, and radiographs were retrospectively reviewed for all patients who experienced a complication within 12 months postsurgery. All complications were identified for analysis. Results Forty-six patients were included in this study; 17 (36.9%) patients had a documented complication. There were 7 patients (15.2%) with loss of correction, 2 (4.3%) with intraoperative lateral cortex fractures, 2 (4.3%) with postoperative lateral cortex fractures, 2 (4.3%) with deep venous thrombosis, 2 (4.3%) with delayed unions, and 2 (4.3%) with symptomatic hardware. Patients with a loss of angular correction had a greater body mass index (BMI) than those without a loss of correction (mean BMI, 32.5 v 28.8; P = .0416). Of the 7 patients with loss of angular correction, 6 had a first-generation fixation device and 1 had a second-generation device. There was no apparent association between delayed union and graft type (allograft v autograft). Conclusions The medial opening wedge HTO is associated with a moderate frequency of complications. The frequency and type of complications seem to be similar to those reported for the lateral closing technique. Although there are technical advantages offered by the medial opening wedge HTO, their influence on the frequency and type of complications experienced by patients in our series was not apparent. Level of Evidence Level IV, case series.

Each year, more than 30 million children and adolescents participate in organized sports in the United States (US), and more than 4.3 million nonfatal sports- and recreation-related injuries are treated in US hospital emergency departments. Two types of activity-related injuries, in particular, that we study in the Bone and Joint Injury Prevention and Rehabilitation Center at the University of Michigan are anterior cruciate ligament (ACL) injuries and head injury and concussion. Knee ACL injuries are occurring in epidemic-like proportions. In the US alone there are approximately 200,000 new ACL injuries at a total cost of over $1 billion per year. ACL-injured patients are at a significant risk of developing early onset osteoarthritis. Exacerbating the concern is the increase in ACL injury rates in youth and adolescents; particularly girls can experience ACL injury rates from two to five times greater than boys in the same sport. In addition, 3.8 million sports- and recreation-related concussions occur in the US each year. Significantly more research is needed, but intriguing findings are surfacing. For example, high school athletes’ recovery times for a concussion are longer than college athletes’ recovery times. Moreover, high school athletes who sustain a concussion are 3 times more likely to sustain a second concussion. Preliminary studies state suggest women have a higher risk for head injury and post concussion syndrome due to potentially weaker neck muscles, different muscle activation patterns, and hormone variations. Three key questions must be answered to understand these and other sports-related injuries: 1) What is the magnitude of the epidemic? 2) What are the causes of injury? and 3) What can we do to prevent injuries? Without a greater awareness of the injury risk and a better understanding of the injury mechanisms, the risk of sport-related injuries may escalate in the coming years. Participation in sport can be beneficial for adults and children to promote exercise and healthy lifestyles. The benefits of exercise include decreasing the incidence of cardiovascular disease, obesity, osteoporosis, and diabetes and fostering a healthy image for children and youth. Although exercise and physical activity can have major benefits, some disadvantages do exist, including the

Context:Sex differences in neuromuscular control of the lower extremity have been identified as a potential cause for the greater incidence of anterior cruciate ligament (ACL) injuries in female athletes compared with male athletes. Women tend to land in greater knee valgus with higher abduction loads than men. Because knee abduction loads increase ACL strain, the inability to minimize these loads may lead to ACL failure.Objective:To investigate the activation patterns of the quadriceps and hamstrings muscles with respect to the peak knee abduction moment.Design:Cross-sectional study.Setting:Neuromuscular research laboratory.Patients or Other Participants:Twenty-one recreationally active adults (11 women, 10 men).Main Outcome Measure(s):Volunteers performed 3 trials of a 100-cm forward hop. During the hop task, we recorded surface electromyographic data from the medial and lateral hamstrings and quadriceps and recorded lower extremity kinematics and kinetics. Lateral and medial quadriceps-to-hamstrings (Q∶H) cocontraction indices, the ratio of medial-to-lateral Q∶H cocontraction, normalized root mean square electromyographic data for medial and lateral quadriceps and hamstrings, and peak knee abduction moment were calculated and used in data analyses.Results:Overall cocontraction was lower in women than in men, whereas activation was lower in the medial than in the lateral musculature in both sexes (P < .05). The medial Q∶H cocontraction index (R2 = 0.792) accounted for a significant portion of the variance in the peak knee abduction moment in women (P = .001). Women demonstrated less activation in the vastus medialis than in the vastus lateralis (P = .49) and less activation in the medial hamstrings than in the lateral hamstrings (P = .01).Conclusions:Medial-to-lateral Q∶H cocontraction appears to be unbalanced in women, which may limit their ability to resist abduction loads. Because higher abduction loads increase strain on the ACL, restoring medial-to-lateral Q∶H cocontraction balance in women may help reduce ACL injury risk.

A patient's access to healthcare resources often begins with scheduling an appointment with a medical doctor or other provider. An inefficient scheduling system leads to unnecessary delays in providing care for patients and frustration for referring physicians. We used the tools of lean thinking to evaluate our current scheduling system, remove wasteful processes and procedures, and implement a more efficient and effective system. In doing so, we increased the value for our patients, who benefit from more timely access to care and greater satisfaction.

Soft-tissue allografts are valuable options in knee ligament reconstructive surgery. The purpose of this study was to determine the risk of soft-tissue contamination before implantation and the occurrence of infection after implantation in patients who received soft-tissue allografts for knee reconstructive procedures. A retrospective review of medical records was performed for patients who had undergone knee ligament surgery with allograft tissues at one institution between 1993 and 2004. Cultures were positive in 6 (5.7%) of 105 cases. Coagulase-negative Staphylococcus was the most common organism. None of these patients developed postoperative infections. The culture-positive group had a longer period of joint effusion postoperatively, compared with the culture-negative group (14.2 weeks versus 9.6 weeks). Patients with positive cultures required no additional treatment other than close observation.

Valgus knee angle (VKA) maybe a predictor of non-contact anterior cruciate ligament (ACL) injuries. Pre-programmed muscle activation strategies may exist which could contribute to the larger VKA displayed in women compared to men. The current study examined the relationship between the peak VKA and preparatory muscle activity. Twenty-one adults were asked to perform five trials of a forward hop. Lower extremity kinematics and surface EMG were recorded. Peak VKA and EMG from 100ms prior to ground contact were used in the data analyses. Three multiple linear regressions, where muscle activity was regressed upon the peak VKA, were run using subsets (female, male, and male/female) of the sample. Partial regression coefficients were considered significant at P ⩽0.05. When female subjects were exclusively included in the model, a higher peak VKA was associated with increased preparatory vastus lateralis and lateral hamstring activity, while a lower VKA was associated with increased preparatory vastus medialis activity ( P ⩽0.05). When both genders and males alone were considered, preparatory activity was not associated with peak VKA ( P ⩾0.05). Neuromuscular training promoting equal preparatory muscle activity in the medial-to-lateral quadriceps and hamstrings may reduce the incidence of ACL injuries in females.

Background Recently published studies have raised the question of whether arteriography is warranted in the evaluation of multiligamentous injuries of the knee. Purpose The objective is to report the frequency of associated vascular injuries in the multiligament-injured knee and examine the role arteriography plays in the treatment protocol. Study Design Case series; Level of evidence, 4. Methods A retrospective analysis was performed on 71 patients over a 12-year period who had a diagnosis of multiligamentous injury of the knee with a tibial-femoral dislocation documented based on physical examination and magnetic resonance imaging findings. Results Of 72 knee injuries involving multiple ligaments, 12 vascular injuries were identified. Four knees were found to have a vascular injury at initial presentation based on abnormal physical examination and confirmed with arteriography. Eight patients with a vascular injury had normal pulses. Routine arteriography discovered an intimal injury of the popliteal artery in 5 of these patients. Arteriography in the remaining 3 patients was interpreted as normal. Conclusion These findings suggest that physical examination alone is not sufficient in detecting the majority of vascular injuries after a suspected knee dislocation.

Background: The hamstring muscles are well positioned to limit both anterior tibial translation and anterior cruciate ligament strain during the knee flexion phase of a jump landing. We hypothesized that systematically increasing or decreasing hamstring tension during the knee flexion phase of a simulated jump landing would significantly affect peak relative strain in the anterior cruciate ligament. Methods: Ten cadaveric knees from four male and six female donors (mean age [and standard deviation] at the time of death, 60.3 ± 23.6 years) were mounted in a custom fixture to initially position the specimen in 25° of knee flexion and simulate axial impulsive loading averaging 1700 N to cause an increase in knee flexion. Quadriceps, hamstring, and gastrocnemius muscle forces were simulated with use of pretensioned linear springs, with the tension in the hamstrings arranged to be increased, held constant, decreased, at “baseline,” or absent during knee flexion. Impulsive loading applied along the tibia and femur was monitored with use of triaxial load transducers, while uniaxial load cells monitored quadriceps and medial and lateral hamstring forces. Relative strain in the anterior cruciate ligament was measured with use of a differential variable reluctance transducer, and tibiofemoral kinematics were measured optoelectronically. For each specimen, anterior cruciate ligament strains were recorded over eighty impact trials: ten preconditioning trials, ten “baseline” trials involving decreasing hamstring tension performed before and after three sets of ten trials conducted with increasing hamstring tension, constant hamstring tension, or no hamstring tension. Peak relative strains in the anterior cruciate ligament were normalized for comparison across specimens. Results: Increasing hamstring force during the knee flexion landing phase decreased the peak relative strain in the anterior cruciate ligament by >70% compared with the baseline condition (p = 0.005). Neither a constant hamstring muscle force nor the absence of a hamstring force significantly changed the peak strain in the anterior cruciate ligament relative to the baseline condition. Conclusions: Increasing hamstring muscle force during the knee flexion phase of a simulated jump landing significantly reduces the peak relative strain in the anterior cruciate ligament in vitro. Clinical Relevance: It may be possible to proactively limit peak anterior cruciate ligament strain during the knee flexion phase of jump landings by accentuating hip flexion, thereby increasing the tension in active hamstring muscles by lengthening them.