Search

Background: Minimal clinically important differences (MCIDs) for different patient outcome scores have been reported for various shoulder diseases, including shoulder arthroplasty and the nonoperative treatment of rotator cuff disease. The purpose of this study was to assess the MCID for the American Shoulder and Elbow Surgeons Standardized Shoulder Assessment Form (ASES) score, the Simple Shoulder Test (SST), and a visual analog scale (VAS) measuring pain, after arthroscopic rotator cuff repair., Methods: A total of 202 patients who underwent arthroscopic rotator cuff repair were retrospectively reviewed. ASES, SST, and VAS pain scores were collected preoperatively and at 1 year postoperatively. The MCID was then calculated via a 4-question anchor-based method., Results: The MCID results for the ASES, SST, and VAS pain scores were 27.1, 4.3, and 2.4, respectively. Age at time of surgery, sex, anteroposterior tear size, and worker's compensation status were not associated with MCID values (P > .05)., Conclusion: The MCID values determined in the current study are higher than those previously identified for the nonoperative treatment of rotator cuff disease using the same anchor questions. Use of these higher values should be considered when evaluating improvements of individual patients after rotator cuff repair, to determine comparative effectiveness of various rotator cuff repair techniques and to determine sample sizes for prospective comparative trials of rotator cuff repair methods., (Copyright © 2019 Journal of Shoulder and Elbow Surgery Board of Trustees. Published by Elsevier Inc. All rights reserved.)

Purpose: To determine, in the context of measuring bone loss in shoulder instability, whether measurement differences between magnetic resonance imaging (MRI) and computed tomography (CT), linear-based and area-based methods, and observers altered the proposed treatment when a standardized algorithm was applied., Methods: This was a retrospective, comparative imaging study of preoperative patients with anterior shoulder instability with both an MRI and CT scan within 1 year of one another. On parasagittal images reoriented en face to the glenoid, 2 attending orthopaedic surgeons measured glenoid width, glenoid area, glenoid defect width, and glenoid defect area. On axial images maximal Hill-Sachs width was measured. From these, linear percent glenoid bone loss (%GBL) and area %GBL were calculated, and on-versus off-track was determined. With these results, a recommended treatment was determined by applying a standardized algorithm, in which the Latarjet procedure was selected for %GBL >20%, arthroscopic labral repair and remplissage for off-track lesions with %GBL <20%, and arthroscopic labral repair on-track shoulders with %GBL <20%., Results: In total, 53 patients with mean ± standard deviation 45 ± 83 days between scans were include with a CT linear %GBL of 23.5 ± 9.6% (range 0%-47%). CT lead to larger measurements of %GBL than MRI (linear P = .008, area P = .003), and fewer shoulders being considered on-track (33.0% vs 40.5%), which would alter treatment in 25% to 34%. Linear measurements produced larger values for %GBL (CT, P < .001; MRI, P < .001), which would alter treatment in 25%. For %GBL, inter-rater reliability was good, with intraclass correlation coefficients varying from 0.727 to 0.832 and Kappa varying from 0.57 to 0.62, but these inter-rater differences would alter treatment in 31%., Conclusions: The significant differences in bone loss measurement between imaging modality, measurement method, and observers may lead to differences in treatment in up to 34% of cases. Linear CT measurements resulted in the most aggressive treatment recommendations., Level of Evidence: Retrospective Comparative Study: Diagnostic, Level III., (Copyright © 2019 Arthroscopy Association of North America. Published by Elsevier Inc. All rights reserved.)

Objectives: To analyze the functional and radiographic outcomes of anatomic coracoclavicular ligament reconstruction (ACCR) using allograft tendon without interference screw fixation., Design: Retrospective nonrandomized study., Setting: Level I trauma center (University Hospital)., Patients: Seventeen patients (mean age of 44 years) with Rockwood III through V acromioclavicular joint disruptions. Twelve of 17 patients had a primary reconstruction, including 4 patients sustaining their injuries as part of a polytrauma incident. Five of 17 patients were revisions of a previously failed acromioclavicular reconstruction procedure., Intervention: Open ACCR using hamstring allograft with high-strength suture augmentation and knotted graft fixation without interference screws., Main Outcome Measures: Clinical and patient reported outcome measures including Simple Shoulder Test, American Shoulder and Elbow Surgeons scores, and visual analog scale scores for pain and radiographic outcomes., Results: The average final postoperative Simple Shoulder Test and American Shoulder and Elbow Surgeons scores were 10.8 and 80.5, respectively. The average final postoperative visual analog scale pain was 1.8. All patients demonstrated clavicle tunnel widening on final postoperative radiographs compared with immediate postoperative radiographs. The overall complication rate was 36%, with no clavicle or coracoid fractures., Conclusions: Open ACCR using hamstring allograft tendon secured with a square knot and high-strength suture augmentation yields equivalent outcomes to those repairs requiring an additional interference screw. Clavicle tunnel widening predictably occurs, but the clinical significance is undetermined. It appears therefore that an interference screw is not needed. Larger comparison studies are needed., Level of Evidence: Therapeutic Level IV. See Instructions for Authors for a complete description of levels of evidence.

Ulnohumeral arthroplasty (UHA) is considered a satisfactory surgical treatment option for patients with primary degenerative arthritis of the elbow. Most series have used categoric elbow scoring systems to evaluate the outcome of this procedure. The purpose of our study was to evaluate the outcome of UHA with patient-derived functional and general health status outcome instruments. We evaluated 17 patients (18 elbows) with primary degenerative arthritis of the elbow at a mean of 85 months after UHA. The mean age at the time of surgery was 42 years (range, 26 to 58 years). At the follow-up evaluation, the patients were assessed with a physical examination, outcome assessment tools, and plain radiographs. The mean elbow flexion arc improved by 16 degrees (range, -15 degrees to 60 degrees ; P = .012), and the mean forearm rotation arc (supination/pronation) improved by 35 degrees (range, -20 degrees to 90 degrees ; P < .001). Of the elbows, 11 were painless, 4 were painful with motion, and 3 were painful at rest and with motion. The mean Hospital for Special Surgery elbow score was 70 (range, 40 to 99), and the mean Mayo Elbow Performance Score was 83 (range, 50 to 100). The mean Disabilities of the Arm, Shoulder and Hand score was 9.75 (range, 0 to 43.48). The Short Form-36 scores were better than the mean age- and sex-adjusted normal values. Patient self-assessed outcomes and general health status after UHA for primary degenerative elbow arthritis appear to be better than those determined by some categoric scoring systems. Consequently, the clinical utility of UHA may be underrepresented if physician-derived categoric scoring systems are the only measures of outcome assessment.

Both inlay and onlay humeral implants are available for reverse total shoulder arthroplasty (rTSA), but biomechanical data comparing these components remain limited. This study investigated the effects of inlay and onlay rTSA humeral components on shoulder biomechanics using a biorobotic shoulder simulator. Twenty fresh-frozen cadaveric shoulders were tested before and after rTSA with either an inlay or onlay humeral implant. Comparisons were performed between the most commonly implanted configurations for each implant (baseline) and with a modification to provide equivalent neck-shaft angles (NSAs) for the inlay and onlay configurations. Specimens underwent passive range-of-motion (ROM) assessment with the scapula held static, and scapular-plane abduction was performed, driven by previously collected human-subject scapulothoracic and glenohumeral kinematics. Passive ROM glenohumeral joint angles were compared using t tests, whereas muscle force and excursion data during scapular-plane elevation were evaluated with statistical parametric mapping and t tests. Maximum passive elevation was reduced for the inlay vs. onlay humeral components, although both implants caused reduced passive elevation vs. the native joint. Inlay rTSA also demonstrated reduced passive internal rotation at rest and increased external rotation at 90° of humerothoracic elevation vs. the native joint. All preoperative planning estimates of ROM differed from experiments. Rotator cuff forces were elevated with an onlay vs. inlay humeral implant, but simulated muscle excursions did not differ between systems. Compared with the native joint, rotator cuff forces were increased for both inlay and onlay implants and deltoid forces were reduced for inlay implants. Muscle excursions were dramatically altered by rTSA vs. the native joint. Comparisons of inlay and onlay humeral implants with equivalent NSAs were consistent with the baseline comparisons. Rotator cuff forces required to perform scapular-plane abduction increase following rTSA using both inlay and onlay implants. Rotator cuff forces are lower with inlay implants compared with onlay implants, although inlay implants also result in reduced passive-elevation ROM. Deltoid forces are lower with inlay implants in comparison to the native joint but not with onlay implants. The differences between inlay and onlay components are largely unaffected by NSA, indicating that these differences are inherent to the inlay and onlay designs. In those patients with an intact rotator cuff, decreased rotator cuff forces to perform abduction with an inlay humeral implant compared with an onlay implant may promote improved long-term outcomes owing to reduced deltoid muscle fatigue when using an inlay implant. [ABSTRACT FROM AUTHOR]

Although typically favorable in outcome, anatomic total shoulder arthroplasty (aTSA) can require long-term revision. The most common cause for revision is glenoid loosening, which may result from eccentric cyclic forces and joint translations. "Rocking" of the glenoid component may be exacerbated by the joint geometry, such as glenoid inclination and version. Restoration of premorbid glenoid inclination may be preferable, although laboratory and computational models indicate that both superior inclination and inferior inclination have benefits. This discrepancy may arise because previous studies were limited by a lack of physiological conditions to test inclination. Therefore, a cadaveric shoulder simulator with 3-dimensional human motion was used to study joint contact and muscle forces with isolated changes in glenoid inclination. Eight human cadaveric shoulders were tested before and after aTSA. Scapular-plane abduction kinematics from human subjects were used to drive a cadaveric shoulder simulator with 3-dimensional scapulothoracic and glenohumeral motion. Glenoid inclination was varied from −10° to +20°, whereas compressive, superior-inferior shear, and anterior-posterior shear forces were collected with a 6- df load cell during motion. Outputs also included muscle forces of the deltoid and rotator cuff. Data were evaluated with statistical parametric mapping repeated-measures analysis of variance and t tests. Inferior glenoid inclination (−10°) reduced both compressive and superior-inferior shear forces vs. neutral 0° inclination by up to 40%, and even more when compared with superior inclination (P <.001). Superior inclinations (+10° and +20°) tended to increase deltoid and rotator cuff forces vs. neutral 0° inclination or inferior inclination, on the order of 20%-40% (P ≤.045). All force metrics except anterior-posterior shear were lowest for inferior inclination. Most aTSA muscle forces for neutral 0° inclination were not significantly different from native shoulders and decreased 45% and 15% in the posterior deltoid and supraspinatus, respectively (P ≤.003). Joint translations were similar to prior reports in aTSA patients and did not differ between any inclinations or compared with native shoulders. Joint reaction forces were similar to those observed in human subjects with instrumented aTSA implants, providing confidence in the relative magnitude of our results. Inferior inclination reduces overall forces in the shoulder. Superior inclinations increase the muscle effort required for the shoulder to achieve similar motion, thus increasing the forces exerted on the glenoid component. These results suggest that a preference toward aTSA glenoid components in inferior inclination may reduce the likelihood of glenoid loosening by reducing excessive muscle and joint contact forces. [ABSTRACT FROM AUTHOR]

Background: Humeral bone loss is commonly encountered during revision shoulder arthroplasty and anticipating humeral bone defects can help the revision surgeon make appropriate plans to achieve adequate fixation and stability. No validated classification system exists to characterize humeral bone loss in the setting of revision shoulder arthroplasty.Questions/purposes: The purposes of this study were (1) to create a classification system for humeral bone loss in revision shoulder arthroplasty; (2) to determine the classification system's reliability; and (3) to determine whether humeral bone loss type is associated with intraoperative humeral-related reconstruction characteristics.Methods: This was a comparative retrospective radiographic study. First, six surgeons from five centers collaborated to create a classification by consensus. Second, two surgeons from two other centers who had fellowship training in shoulder and elbow surgery, who were blinded to each other's grades and all patient details other than plain radiographs, and who were not involved in creation of the system, classified true AP, AP, and lateral (axillary and/or scapular-Y) radiographs from 108 revision (413 radiographs) from one center that were performed between November 15, 2006, and January 4, 2018. Interobserver reliability was calculated by comparing those two reviews and determining Cohen's κ. In addition, one reviewer repeated his assessments twice, 4 months apart, to determine intraobserver reliability using Cohen's κ. Third, we performed a retrospective chart study of these same revisions to determine intraoperative humeral-related reconstruction characteristics such as the use of greater tuberosity fixation, stem length, humeral bone grafting, and the use of proximal humeral replacement or total humeral replacement; at the center where these revisions were performed during that timespan, no attempt to classify bone loss was made. During that period, the general indications for greater tuberosity fixation included the absence of a stable osseous connection between the greater tuberosity and the shaft of the humerus with a tuberosity amendable to repair; the general indications for use of longer stems were inability to obtain a minimum of two cortical widths of overlap between the implant and the humeral diaphysis and/or loss of the greater tuberosity; and the general indications for proximal and total humeral replacement were bone loss that was felt to be too severe to allow reconstruction with allograft.Results: The classification system consists of three types of humeral bone loss: Type 1 is loss of the epiphysis with subtypes for loss of the calcar and loss of the greater tuberosity; Type 2 is loss of the metadiaphysis above the deltoid attachment with a subtype for cortical thinning; and Type 3 is bone loss extending below the deltoid attachment with a subtype for cortical thinning. We studied 108 revisions: 38 (35%) without bone loss, 34 (31%) Type 1, 27 (25%) Type 2, and nine (8%) Type 3. For reliability, interrater κ was 0.545 and in 71% (77 of 108) of revisions, the two raters agreed on a numeric type. Intrarater κ was 0.615 and in 77% (83 of 108) of revisions, the rater agreed with himself as to the numeric type. Stem length increased with class type (Type 1 median [range] 130 [70-210], Type 2 150 [70-210], Type 3 190 [70-240], p = 0.005). Most greater tuberosity fixation for intraoperative fracture was in Types 1 and 2 (13 of 18 compared with the five of 18 of greater tuberosity fixation that was within Types 0 and 3, p = 0.043). Most bone grafting was in Types 2 and 3 (eight of 13 compared with five of 13 of bone grafting was in Types 0 and 1, p = 0.044). Most proximal humeral and total humeral replacements were in Type 3 (three of four compared with one of four, p < 0.001).Conclusions: We developed the Proximal Humeral Arthroplasty Revision Osseous inSufficiency (PHAROS) system, which has adequate, if imperfect, reliability to classify humeral bone loss in the setting of revision shoulder arthroplasty. This classification system may be useful to anticipate the complexity of humeral reconstruction. Further validation incorporating advanced imaging and further evaluators will be necessary.Level Of Evidence: Level III, diagnostic study. [ABSTRACT FROM AUTHOR]

The article discusses several studies related to shoulder and elbow injury from April 2017 to March 2018. These studies include one on radiographic progression of primary osteoarthritis, another on patients who underwent anatomic total shoulder arthroplasty using a Trabecular Metal-backed glenoid component, and one on shoulder arthroplasty patients with regard to the factors associated with the development of deep operative infection.

Shoulder arthroplasty has advanced considerably in recent decades, but the most recently published long-term outcomes continue to have unacceptably high failure rates, especially in young patients. However, there are several future frontiers that offer the promise of improved implant longevity and function including advances in surgical approach, such as subscapularis-sparing approaches and computer-assisted surgical planning; advances in humeral component fixation, such as stemless, short-stem, and convertible implants; advances in bearing surfaces, such as pyrocarbon, ceramic, and metal-on-metal; and advances in glenoid component geometry and fixation, such as augmented components, in-growth pegs, and inlay glenoid components. [ABSTRACT FROM AUTHOR]

Background: Instability is the most common complication after reverse total shoulder arthroplasty (rTSA). In the native glenohumeral joint, in addition to full dislocations, more subtle forms of instability exist. However, the incidence of more subtle forms of instability, the factors associated with instability, and the effect of instability on validated outcome scores after rTSA remain poorly understood.Questions/purposes: (1) After rTSA, what is the risk of instability, including more subtle forms of subjective instability? (2) What are the factors associated with instability? (3) Are more subtle forms of instability associated with lower American Shoulder and Elbow Surgeons (ASES) functional outcome scores than those patients without instability?Methods: A total of 168 rTSAs were performed during the study period. Six patients had died at the time of study initiation. Thirty patients were excluded, nine because rTSA was performed for an acute proximal humeral fracture, one because a lateralized humeral component was used, 17 because a retaining liner was used, and three because a lateralized glenosphere was used. One hundred thirty-two patients met inclusion and exclusion criteria. Thirty-five patients were lost to followup. Thus, 97 patients with a minimum of 2 years followup were included in the final cohort (74% of included patients). Followup was 47 ± 22 months (mean ± SD). The cohort included 23 men and 74 women with an age of 70 ± 9 years who underwent 78 primary and 19 revision rTSAs. Primary and revision patients were combined for subsequent analyses. A postoperative questionnaire was used to assess instability symptoms. Although it has not been validated, it is simple and we believe has high face validity. Briefly, it scored instability as (1) none; (2) feelings of instability; (3) probable dislocation/subluxation-self-reduced; and (4) dislocation with surgical reduction or dislocation with closed reduction (such as in the emergency department or the doctor's office). ASES scores were collected specifically for this study. The preoperative and postoperative β angle was measured to determine glenoid inclination. Larger β angles denote more superior inclination, whereas smaller β angles denote more inferior inclination. Thus, a positive change in β angle from preoperatively to postoperatively denotes a change into more superior inclination, whereas a negative change in β angle from preoperatively to postoperatively denotes a change into more inferior inclination. Associations between instability symptoms and patient, implant, and surgical factors were evaluated in a multivariate model that considered age, sex, body mass index, and whether it was a primary or a revision procedure.Results: A total of 13 of 97 (13%) patients reported some instability (Grades 2-4); four of 97 patients (4%) had full dislocations with reduction (Grade 4), four of 97 patients (4%) reported subluxations (Grade 3), and five of 97 patients (5%) reported feelings of instability or apprehension (Grade 2). After controlling for potential confounding variables like age, sex, body mass index, and revision versus primary procedure, the only factors associated with instability were greater superior baseplate inclination (larger β angle; odds ratio [OR], 1.15 [95% confidence interval {CI}, 1.042-1.258]; p = 0.005) and a greater change into superior inclination from preoperative to postoperative (greater positive change in ß angle; OR, 1.08 [1.009-1.165]; p = 0.027). Patients with any instability (Grades 2-4) reported lower final ASES scores than did patients without instability (Grade 1) (61 ± 16 versus 72 ± 19 mean difference 11 [95% CI, 0-22]; p = 0.032).Conclusions: When more subtle instability after rTSA is included, instability may occur in up to 13% of patients. Instability is associated with greater superior baseplate inclination and less inferior correction of the β angle and thus surgeons should consider inferiorly inclining the baseplate to avoid postoperative instability. Although our study only demonstrates an association and not causation, the authors hypothesize that superior baseplate inclination increases inferior impingement, which leads to instability. Instability negatively influences final ASES score.Level Of Evidence: Level III, therapeutic study. [ABSTRACT FROM AUTHOR]

Background The best chance that a shoulder arthroplasty will restore motion and muscle balance across the glenohumeral joint is by closely replicating natural articular morphology. Defining the humeral osteotomy plane along clear landmarks at the anatomic neck is critical. We hypothesized that a new osteotomy, based on alternative landmarks on the anatomic neck, would restore 3-dimensional humeral head morphology more reliably than the traditional osteotomy. Methods The anatomic neck was digitized in 30 human cadaver shoulders and compared with its 3-dimensional computed tomography reconstruction. Two different osteotomy techniques were virtually performed: the traditional, following the anterosuperior anatomic neck; and a new technique, defined by the inferoanterior anatomic neck. The length-width difference and orientation (retroversion, inclination) of the resection area were compared between the techniques and with native anatomy. Results Length-width difference of the anterosuperior resection area was higher than in the inferoanterior osteotomy (6 ± 2 mm vs. 3 ± 1 mm; P  < .001). Retroversion of the anterosuperior resection plane was higher than the native head (50° ± 12° vs. 37° ± 11°; P  < .001), whereas retroversion after the inferoanterior osteotomy (32° ± 12°) did not differ from native ( P  = .057). Inclination differed after the anterosuperior osteotomy (129° ± 5°) and the inferoanterior osteotomy (127° ± 4°) compared with the native head (134° ± 4°; P  ≤ .001). Conclusion The inferoanterior referenced osteotomy generated a more circular resection area, matching the native humeral head retroversion more closely than in the anterosuperior technique. This study suggests that in shoulder arthroplasty, the humeral resection level should be referenced at the inferoanterior rather than the anterosuperior anatomic neck. Further studies should investigate the biomechanical effects of this alternative resection plane. [ABSTRACT FROM AUTHOR]

Background: Studies have reported high rates of transfusion in shoulder arthroplasty. This study was conducted to evaluate the rate of transfusion at our institution, to confirm reported risk factors for transfusion, and to look for changes over time.We hypothesized that transfusion rates associated with shoulder arthroplasty at our institution are lower than those recently reported and that the incidence of transfusion is higher in individuals with low preoperative hemoglobin, with revision arthroplasty, and in older individuals. Materials and methods: A retrospective review of 366 shoulder arthroplasties (323 patients) was performed. This included total shoulder arthroplasties, hemiarthroplasties, revision arthroplasties, and reverse total shoulder arthroplasties. Logistic regression analysis evaluated the association of clinical variables with transfusion. Early (1996-2005) and late (2006-2009) groups were compared to evaluate changes in demographics and transfusion rates over time. Results: The overall transfusion rate was 7.4% (27 of 339). Predictors of transfusion were higher intraoperative blood loss, low preoperative hemoglobin level, and humeral cement fixation. Procedure type was not predictive of transfusion. There was no difference in transfusion rates between the early and late groups, but the late group had an increased use of general anesthesia combined with a regional block, increased intraoperative blood loss, and increased use of sequential compression devices for venous thromboembolism prophylaxis. Conclusions: Lower preoperative hemoglobin, higher intraoperative blood loss, and humeral cement fixation were predictors of transfusion, but not female sex, increasing age, type of procedure, or comorbidities. [ABSTRACT FROM AUTHOR]

Background: Lateral offset center of rotation (COR) reduces the incidence of scapular notching and potentially increases external rotation range of motion (ROM) after reverse total shoulder arthroplasty (rTSA). The purpose of this study was to determine the biomechanical effects of changing COR on abduction and external rotation ROM, deltoid abduction force, and joint stability. Materials and methods: A biomechanical shoulder simulator tested cadaveric shoulders before and after rTSA. Spacers shifted the COR laterally from baseline rTSA by 5, 10, and 15 mm. Outcome measures of resting abduction and external rotation ROM, and abduction and dislocation (lateral and anterior) forces were recorded. Results: Resting abduction increased 20° vs native shoulders and was unaffected by COR lateralization. External rotation decreased after rTSA and was unaffected by COR lateralization. The deltoid force required for abduction significantly decreased 25% from native to baseline rTSA. COR lateralization progressively eliminated this mechanical advantage. Lateral dislocation required significantly less force than anterior dislocation after rTSA, and both dislocation forces increased with lateralization of the COR. Conclusion: COR lateralization had no influence on ROM (adduction or external rotation) but significantly increased abduction and dislocation forces. This suggests the lower incidence of scapular notching may not be related to the amount of adduction deficit after lateral offset rTSA but may arise from limited impingement of the humeral component on the lateral scapula due to a change in joint geometry. Lateralization provides the benefit of increased joint stability, but at the cost of increasing deltoid abduction forces. [Copyright &y& Elsevier]

Background: No clear recommendations exist regarding optimal humeral component version and deltoid tension in reverse total shoulder arthroplasty (TSA). Materials and methods: A biomechanical shoulder simulator tested humeral versions (0°, 10°, 20° retroversion) and implant thicknesses (−3, 0, +3 mm from baseline) after reverse TSA in human cadavers. Abduction and external rotation ranges of motion as well as abduction and dislocation forces were quantified for native arms and arms implanted with 9 combinations of humeral version and implant thickness. Results: Resting abduction angles increased significantly (up to 30°) after reverse TSA compared with native shoulders. With constant posterior cuff loads, native arms externally rotated 20°, whereas no external rotation occurred in implanted arms (20° net internal rotation). Humeral version did not affect rotational range of motion but did alter resting abduction. Abduction forces decreased 30% vs native shoulders but did not change when version or implant thickness was altered. Humeral center of rotation was shifted 17 mm medially and 12 mm inferiorly after implantation. The force required for lateral dislocation was 60% less than anterior and was not affected by implant thickness or version. Conclusion: Reverse TSA reduced abduction forces compared with native shoulders and resulted in limited external rotation and abduction ranges of motion. Because abduction force was reduced for all implants, the choice of humeral version and implant thickness should focus on range of motion. Lateral dislocation forces were less than anterior forces; thus, levering and inferior/posterior impingement may be a more probable basis for dislocation (laterally) than anteriorly directed forces. [ABSTRACT FROM AUTHOR]