Search

Anterior cruciate ligament injury affects roughly 120 000 athletes in the United States every year. One of the most common techniques is the use of a bone-patellar tendon-bone graft. Graft harvest creates a sizeable defect in the remaining patellar tendon. Closure of this defect is based on surgeon preference. To date there has been no study on the effects of defect closure on the mechanical properties of remaining donor patellar tendon. The goal of this study was to investigate the effect of closure on both the strength and stiffness of the remaining patellar tendon. 7 pairs of fresh frozen cadaver patellar tendons were matched by tendon dimensions. Bone-patellar tendon-bone grafts were harvested from all of the specimens and then half of the paired tendons underwent defect closure. All of the donor tendons were then tested in a servohydraulic load frame to failure at a constant displacement rate at room temperature. This study found no differences in the load at failure, the engineering failure stress, stiffness or in the engineering modulus between the donor tendons that underwent defect closure versus those that did not. [ABSTRACT FROM AUTHOR]

It has been shown that preheating the femoral stem prior to insertion minimizes interfacial porosity at the stem-cement interface. In this study, the effects of methylmethacrylate monomer temperature prior to mixing on the properties of stem-cement interface and cement polymerization were evaluated for 4 degrees C, room temperature, and 37 degrees C using a test model and cementing techniques that simulated a clinical situation. The nature and extent of interfacial porosity of stem-cement interface was quantified, the static shear strength of the stem-cement interface determined, and the time and temperature of polymerization at the cement-bone interface were measured. Compared to RT monomer, preheating monomer to 37 degrees C produced higher polymerization temperatures and greater initial interfacial shear strength with an unchanged amount of interfacial porosity. Precooling monomer to 4 degrees C produced lower polymerization temperatures and decreased initial interfacial shear strength, with the amount of interfacial porosity unchanged compared to the RT group. Although clinical techniques of preheating or precooling bone cement have some effects on the properties of the stem-cement interface and cement polymerization, they do not appear to enhance implant fixation., (Copyright 2003 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater 68B: 186-190, 2004)

Study Design: A biomechanical study to evaluate the effects of a biodegradable calcium phosphate (Ca-P) bone substitute on the fixation strength and bending rigidity of vertebral body screws., Objectives: To determine if an injectable, biodegradable Ca-P bone substitute provides significant augmentation of anterior vertebral screw fixation in the osteoporotic spine., Summary of Background Data: Polymethylmethacrylate (PMMA) augmented screws have been used clinically; however, there is concern about thermal damage to the neural elements during polymerization of the PMMA as well as its negative effects on bone remodeling. Injectable, biodegradable Ca-P bone substitutes have shown enhanced fixation of pedicle screws., Methods: Sixteen fresh cadaveric thoracolumbar vertebrae were randomly divided into two groups: control (no augmentation) (n = 8) and Ca-P bone substitute augmentation (n = 8) groups. Bone-screw fixation rigidity in bending was determined initially and after 10(5) cycles, followed by pullout testing of the screw to failure to determine pullout strength and stiffness., Results: The bone-screw bending rigidity for the Ca-P bone substitute group was significantly greater than the control group, initially (58%) and after cyclic loading (125%). The pullout strength for Ca-P bone substitute group (1848 +/- 166 N) was significantly greater than the control group (665 +/- 92 N) (P < 0.01). Stiffness in pullout for the Ca-P bone substitute groups (399 +/- 69 N/mm) was significantly higher than the control group (210 +/- 51 N/mm) (P < 0.01)., Conclusion: This study demonstrated that augmentation of anterior vertebral body screw fixation with a biodegradable Ca-P bone substitute is a potential alternative to the use of PMMA cement.

The effect of stem length, diameter, and mode of fixation on the motion and stress transfer of a cemented tibial tray were evaluated for in 12 cadaver knees. There was a significant decrease in motion of the tibial tray with increasing press-fit stem length (75-150 mm) and increasing stem diameter (10-14 mm). Cemented tibial stems showed significantly less tray motion than uncemented stems. The short cemented stems produced tray stability equivalent to long press-fit stems. Although there was a trend for increased proximal tibial stress shielding with the use of cement and longer, wider stems, the trend was not statistically significant. Modular, press-fit stems can achieve tray stability similar to a smaller cemented stem and can avoid the potential problems with cement.

Recently, a new, shorter IM nail using two 6 mm reconstruction screws for proximal fixation was introduced in two versions for femoral insertion: piriformis fossa (FAN) and greater trochanter (TAN). These nails were compared experimentally for their fixation stability, proximal load transmission, and failure strength in an unstable intertrochanteric fracture model in cadaveric femurs. Vertical and axial loads were first applied to the intact femurs. Fractures were created, subsequent fixation applied, and the femurs underwent a series of both vertical and axial loading tests. There was no significant difference in strain readings between the nails for either axial loading or cyclical loading. There was no statistically significant difference between the loads to failure for the trochanteric nails and the standard antegrade nails. The average ultimate loadfor the FAN and TAN nails were 3010 N and 2830 N respectively. These two nails performed very similarly throughout our testing.

The ability of various screw types to stabilize depressed tibial plateau fractures was determined in a biomechanical study using a Sawbones model. Two sizes of both cancellous and cortical screws were evaluated for both supportfrom below and through the depressed fragment. As a general trend, cancellous bone screws provided a greater resistance to fragment displacement than cortical bone screws, and screws with a smaller thread diameter provided greater resistance to displacement than screws of the same thread type with a larger diameter. These results agree with the accepted standard that cancellous screws provide better fixation for tibial plateau fractures, but also are counterintuitive in that smaller screws provided greater fixation than larger screws of the same type.

To compare the intrinsic stability of two distal interlocking screw orientations for tibial nailing of distal third tibial diaphyseal fractures without isthmal support, six Depuy (Warsaw, Indiana) tibial intramedullary nails were implanted in simulated distal tibiae. The constructs received both two parallel (medial to lateral) and two perpendicular (one medial to lateral, one anterior to posterior) distal interlocking screws in a random order Angular, translational, and torsional displacements of the nails were measured in response to 70 N proximal applications of anterior, posterior, medial, and lateral loads, and a 7.7 Newton-meter torsional load. There were no differences in medial or lateral angulations between the screw orientations (average: 2.5 degrees, p > 0.8). Angulation in the sagittal plane (anterior and posterior) was slightly less for parallel screw fixation (1.6 degrees versus 2.4 degrees), but this was not statistically significant (p > 0.1). Rotational angulation was higher in the parallel (average: 9.9 degrees) versus the perpendicular (average: 8.1 degrees) screw orientation, but these results were not statistically significant (p > 0.1). Pure translation did not occur in either the parallel or perpendicular screw orientations. These results indicate that fixation stability of these tibial intramedullary nails is not significantly influenced by distal interlocking screw orientation in response to sagittal, coronal, or rotational forces.

Although clinical studies have shown that posterior cut angle affects tibial component stability, biomechanical studies are lacking. Fifteen Sawbones tibiae were divided into 5 groups and prepared with 0 degrees , 3 degrees, 6 degrees, 9 degrees, and -5 degrees tibial surface cuts, and a tibial component was implanted with cement. Using a standard and then a highly congruent polyethylene insert, the knee was loaded at 0 degrees and 30 degrees of flexion. There were statistically significant increases in anterior micromotion of the standard polyethylene component for each increase in posterior slope cut angle, which increased for the highly congruent polyethylene component. The anterior slope cut (-5 degrees) led to significant posterior micromotion of the tibial polyethylene component. Increased posterior slope cut angle significantly decreased tibial anterior compressive strains and significantly increased tibial posterior compressive strains. The highly congruent insert significantly increased this posterior strain. The results indicate that cutting the articular surface of the tibia at a 0 degrees or 3 degrees posterior slope provides the greatest tibial component stability.

Patellofemoral complications, instability, and tibial polyethylene wear after total knee arthroplasty (TKA) resulting from malrotation of the tibial or femoral components (or both) may be difficult to diagnose based on physical examination and standard knee radiographs. The preoperative assessment of implant rotational alignment is critical in planning treatment because the femoral or tibial component (or both) may need to be revised if malpositioned. The purpose of this study was to ascertain the accuracy of computed tomography (CT) scan for determining rotational alignment of femoral and tibial components in TKA. TKA components were inserted in human cadaver specimens at neutral and 5 degrees of external or internal rotation. For each position, the amount of rotation, determined from digital photographs, was compared with CT scan. The correlation coefficient between these two values averaged 0.87, which was significant at P < .05. The CT scan protocol described in this study can be applied clinically to patients with patellofemoral complaints to confirm or rule out the presence of component malrotation.

Schuhli locking nuts can be used in poor quality cortical bone to enhance fixation stability as an alternative to cement augmented screws. This study compared the fixation strength and stability of plate constructs using Schuhli locking nuts with standard screws and cement augmented screws for fixation of simulated humeral shaft fractures in a test model with osteoporosis. The constructs were tested in axial compression, 4-point bending, and torsion to determine fixation stability. The humeri were cycled in torsion (4.5 Nm) for 1000 cycles to simulate upper extremity use during the early postoperative period and retested for stability. The Schuhli locking nuts and cement augmented screws had significantly greater fixation stability than the standard screws before (range, 6-14 times greater) and after cycling in torsional loading (range, 3-3.6 times greater). Although cement augmented screws and Schuhli augmentation showed increased fixation stability compared with the standard screws in axial and 4-point bending before cycling (range, 1.3-1.4 times greater), this was not significant. Compared with Schuhli fixation, cement augmented screws showed no significant difference in fixation stability in all loading modes before and after cycling. Schuhli locking nuts offer the stability of cement augmentation while avoiding its potential adverse effects on fracture healing with extravasation and thermal necrosis.

This study evaluated 5 currently used periprosthetic femoral shaft fracture fixation techniques to determine which technique provided the greatest fixation stability. Periprosthetic fractures in 30 synthetic femurs were fixed with a plate with cables, plate with proximal cables and distal bicortical screws (Ogden concept), plate with proximal unicortical screws and distal bicortical screws, plate with proximal unicortical screws and cables and distal bicortical screws, or 2 allograft cortical strut grafts with cables. These specimens were then tested in 3 physiologic loading modes. The plate constructs with proximal unicortical screws and distal bicortical screws or with proximal unicortical screws, proximal cables, and distal bicortical screws were significantly more stable in axial compression, lateral bending, and torsional loading than the other fixation constructs studied.

Although biomechanical data indicates that anterior fixation alone in unstable cervical injuries may not provide adequate stability, reports of clinical series indicate general success with this method of treatment. The specific contribution of posterior column injury to overall stability following reconstruction has not been evaluated. This study examined the biomechanical stability of anterior and/or posterior plate fixation following anterior corpectomy and reconstruction for unstable cervical injuries with varying degrees of posterior element injury. The C4-C6 motion segments of ten fresh frozen bovine cervical spines were used. After mounting, nondestructive mechanical testing in axial compression, torsion, flexion, extension, and lateral bending was done as an intact control. A C5 corpectomy with reconstruction using a synthetic bone graft was performed and the posterior ligaments sectioned at the C5-C6 level. Each specimen was sequentially instrumented with anterior and posterior plating alone and in combination and each construct was mechanically retested. The specimens were then further destabilized by bilateral facetectomies at C5-C6 and again tested with the same instrumentation combinations. In comparison to the controls, the spines with a C5 corpectomy/bone graft and posterior ligament rupture with anterior plating demonstrated significantly increased stiffness in flexion, extension, and lateral bending; posterior plating increased stiffness in only flexion and lateral bending. In axial compression and torsion, anterior or posterior plating demonstrated stiffness similar to the controls. Further destabilization by facetectomy significantly decreased stiffness of the instrumented construct (less than control) in torsion with anterior or posterior plate fixation alone. Combined plating showed increased stability compared to controls in all loading conditions for both patterns of instability. Anterior plating alone was able to restore the stability of the cervical spines with posterior ligamentous injury after corpectomy, but it failed to do so with the addition of bilateral facetectomies. For the unstable cervical spine with significant bilateral loss of posterior bony contact, anterior or posterior plating alone may not provide sufficient stabilization in the absence of any additional external immobilization. Combined plating should be considered, which may obviate the need for external immobilization.

The effects of the anterior and posterior sternoclavicular joint (SCJ) soft tissue structures on joint dislocation strength by sequential sectioning the ligaments and capsule of twenty-eight SCJs were evaluated. The medial clavicle of each specimen was initially loaded in the anterior and posterior directions to provide control values for joint laxity. The anterior or posterior ligaments and capsular structures of the SCJs were then selectively cut and the specimens retested for laxity and then loaded to failure simulating either anterior or posterior dislocation. Testing of intact specimens showed that the posterior ligaments were stiffer than other structures in that it was significantly more difficult to posteriorly displace the SCJ than in any other direction and that the capsule was the important anterior structure affecting joint laxity. Load-to-failure testing showed that it required 50% more force to create a failure by posterior dislocation than by anterior dislocation. The results of this study explain the clinical rarity of posterior sternoclavicular joint dislocations.

Background: A comminuted, intra-articular distal femur fracture was surgically treated by the authors with a locked, double-plating technique because fixation stability could not be initially achieved by using a standard double-plating technique. The purpose of this study was to determine biomechanically whether a locked double-plate construct would enhance fixation stability compared with a nonlocked double-plate construct., Methods: Six matched pairs of mildly osteopenic femurs were selected and all had a reproducible intra-articular fracture pattern created. Each pair underwent fixation with either a double-plating construct or a locked, double-plating construct that was randomly assigned. The instrumented femurs were then mechanically tested in several loading modes to determine fixation stability. After initial testing, specimens were cyclically loaded and retested for stability., Results: The locked, double-plating construct provided significantly greater fixation stability than the standard double-plating construct in precycling and postcycling biomechanical testing., Conclusion: The technique described is particularly applicable for severely comminuted fractures of the distal femur and fractures in osteopenic bone with poor screw purchase. It offers a simple alternative for enhancing fixation stability, which avoids the potential complications of methylmethacrylate-enhanced screw fixation.

Variations in the shape and orientation of the anterior acromion have been implicated as predisposing factors for the development of rotator cuff problems. We determined and analyzed by computer digitization the anterior and posterior acromial slopes for 141 cadaver shoulders (whose rotator cuff status had been previously assessed). No significant differences in either the anterior or posterior angle were found between the intact and rotator cuff tear groups. Frequency histograms of both angles demonstrated continuous, normal distributions. The individual acromions were consistently asymmetric in that the anterior slope was larger than the posterior slope. However, the anterior slope distribution did not reveal groupings indicative of the specific acromial types previously reported. These results suggest that acromial classification into flat, curved, and hooked types does not accurately describe the actual anatomical findings.

A biomechanical cadaver study was performed to compare the strength and stability of three cannulated cancellous lag screws with a sliding hip screw for fixation of a vertically oriented fracture of the femoral neck (Pauwels Type III). Using eight matched pairs of human cadaveric femurs, vertically oriented femoral neck osteotomies were created, reduced, and randomized to one of the two fixation methods. The constructs were tested with incremental axial loading from 100 N to 1200 N and cyclical loading at 1000 N for 10,000 cycles; fracture displacements and ultimate load to failure were determined. The specimens stabilized using a sliding hip screw showed less inferior femoral head displacement, less shearing displacement at the osteotomy site, and a much greater load to failure than did those stabilized with multiple cancellous lag screws. These results support the use of a sliding hip screw for treatment of vertically oriented fractures of the femoral neck.

The heel fat pad is organized, both in structure and in composition, to bear the stresses and strains of normal activities and to permit pain-free weightbearing. The fatty acid composition of heel pads in 11 patients with rheumatoid arthritis, a disease process frequently associated with heel fat pad atrophy, was analyzed using gas-liquid chromatography and was compared with that of patients without systemic disease. The heels of patients with rheumatoid arthritis demonstrated a significant change in the composition of saturated fatty acids when compared with heels of nonrheumatoid patients. This composition reflects an increased fat viscosity, which decreases the ability of the heel to absorb and dissipate the energy generated during ambulation. This factor could cause degeneration of the heel septal system, with resulting fat pad atrophy.

The orientation of the acetabular cup and position of lip augmentation (if present) may improve postoperative total hip replacement stability by decreasing dislocation caused by hip prosthesis impingement during rotation. To determine how these cup parameters affect dislocation, the range and amount of rotation for two standard femoral components was determined in a Sawbones hemipelvis model. The parameters that allowed for maximal range of rotation were a cup angle of inclination between 35 degrees and 45 degrees and cup anteversion between 0 degrees and 10 degrees. Cup anteversion angles greater than 20 degrees and cup angles of inclination greater than 45 degrees significantly limited internal and external rotation, particularly for hip flexion greater than 60 degrees. The position of the cup lip augmentation did not affect the amount of rotation except when the hip was flexed and the lip superiorly oriented. Although there are currently devices to facilitate a particular cup position, hip stem type and orientation are other important factors affecting range of rotation that must be considered.

Although most fractures of the proximal femur result from a fall and are related to direct loads to the hip, there is evidence that intrinsic factors, such as muscle contraction, can result in a hip fracture and subsequent fall. This paper reviews the current literature on the various mechanisms of femoral neck and intertrochanteric fractures.

Laboratory evaluation of a self-compressing tibial nail demonstrated that significant, initial compression of a simulated fracture or nonunion can be obtained and controlled. However, when this nail was tested in cyclic loading, loss of its initial static compression occurred.

We compared two different subtrochanteric fracture fixation techniques, an intramedullary hip screw system (IMHS) and an extramedullary, dual sliding screw-plate system (MSP), to determine relative fixation stability. 6 matched pairs of osteosynthesized osteopenic cadaver femurs were axially loaded to 1000 N with concurrent, simulated abductor forces of 0%, 50%, or 86% of the applied head force. The initial loading sequence was made with uniaxial dynamization--the lag screw of the MSP locked and distal locking of the IMHS nail. Femoral head displacement and medial femoral strain were measured for intact femur controls, after fixation of a 2-part reverse oblique subtrochanteric fracture and finally a 3-part reverse oblique subtrochanteric fracture with a lateral wedge defect. The samples were then loaded at 750 N for 10(4) cycles with both devices uniaxially locked, followed by 10(4) cycles with both devices fully biaxially dynamized (unlocked). For the 2-part subtrochanteric fracture pattern, both devices exhibited similar inferior displacements of the femoral head (average 2.0 mm) and medial femoral strain (approximately 70% of intact). Increasing abductor forces decreased medial compressive strain but did not significantly affect head displacement. For the 3-part fracture model, the MSP demonstrated significantly less inferior displacement of the head (1.6 mm vs. 2.1 mm) and both devices demonstrated significantly decreased medial strain. After cycling, head displacement increased approximately 50% in both devices and medial strain increased slightly. After unlocking and cycling, the MSP group showed significant lateral displacement of the proximal fragment. The IMHS and MSP devices provide similar stability for fixation of 2-part and 3-part reverse oblique subtrochanteric fractures. In a biaxially dynamized, 3-part reverse oblique fracture, displacement of the proximal fragment can occur with the MSP.