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Background: Hill-Sachs defects have been associated with failed repairs for anterior shoulder instability. However, the biomechanical consequences of these defects are not well understood because of the complicated interaction between the passive soft tissue and bony stabilizers.

Hypothesis: The creation of a 25% Hill-Sachs defect would not significantly alter the glenohumeral translations but would increase the in situ forces in the glenohumeral capsule as well as the glenohumeral bony contact forces.

Study Design: Controlled laboratory study.

Methods: A robotic/universal force-moment sensor (UFS) testing system was used to apply joint compression (22 N) and an anterior or posterior load (44 N) to cadaveric shoulders (n = 9) with the skin and deltoid removed (intact) at 3 glenohumeral joint positions (abduction/external rotation): 0°/0°, 30°/30°, and 60°/60° (corresponds to 90°/90° of shoulder abduction/external rotation). A 25% bony defect on the posterolateral humeral head (defect) was then created in the most common position of anterior shoulder dislocation (90°/90°), and the loading protocol was repeated. A nonparametric repeated-measures Friedman test with a Wilcoxon signed-rank post hoc test was performed to compare translations, in situ forces in the capsule, and bony contact forces between each state (P < .05).

Results:At 0°/0°, anterior translation significantly increased from 15.3 ± 8.2 mm to 16.6 ± 9.0 mm (P < .05) in response to an anterior load. At 30°/30°, anterior and posterior translations, respectively, significantly increased in response to both anterior (intact: 13.6 ± 7.1 mm vs defect: 14.2 ± 7 mm; P < .05) and posterior loads (intact: 15.7 ± 5.8 mm vs defect: 17.7 ± 5.1 mm; P < .05). In situ force in the capsule during anterior loading was increased in the defect state at both 60°/60° (intact: 38.9 ± 14.4 N vs defect: 43.2 ± 15.9 N; P < .05) and 30°/30° (intact: 39.6 ± 13.8 N vs defect: 45.6 ± 9.3 N; P < .05). The medial bony contact forces were also increased in the defect state at 30°/30° (intact: 25.0 ± 13.8 N vs defect: 28.9 ± 13.2 N; P < .05) during anterior loading.

Conclusion: We believe that the stabilizing function of the intact capsule was the primary contributor to the finding of only small increases of anterior translation, capsule forces, and bony contact forces observed with a 25% Hill-Sachs defect in response to an anterior load.

Clinical Relevance: These findings imply that a 25% Hill-Sachs defect in isolation may not be responsible for recurrent instability if the function of the capsule is restored to the intact state and that the presence of the Hill-Sachs defect may be a marker for significant concomitant injury to the anterior glenoid rim. However, the small changes in these parameters may have long-term implications for the development of osteoarthritis.

Background: Enhancing anterior-posterior (AP) stability in acromioclavicular (AC) reconstruction may be advantageous.

Purpose: To compare the initial stability of AC reconstructions with and without augmentation by either (1) a novel “reverse” coracoacromial (CA) ligament transfer or (2) an intramedullary AC tendon graft.

Hypothesis: Reverse CA transfer will improve AP stability compared with isolated coracoclavicular (CC) reconstruction.

Study Design: Controlled laboratory study.

Methods: Six matched pairs of cadaveric shoulders underwent distal clavicle resection and CC reconstruction. Displacement (mm) was measured during cyclic loading along AP (±25 N) and superior-inferior (SI; 10-N compression, 70-N tension) axes. Pairs were randomized to receive each augmentation and the same loading protocol applied.

Results: Reverse CA transfer (3.71 ± 1.3 mm, standard error of the mean [SEM]; P = .03) and intramedullary graft (3.41 ± 1.1 mm; P = .03) decreased AP translation compared with CC reconstruction alone. The SI displacement did not differ. Equivalence tests suggest no difference between augmentations in AP or SI restraint.

Conclusion: Addition of either reverse CA transfer or intramedullary graft demonstrates improved AP restraint and provides similar SI stability compared with isolated CC reconstruction.

Clinical Relevance: Reverse CA ligament transfer may be a reasonable alternative to a free tendon graft to augment AP restraint in AC reconstruction.

Background: Despite the attention that has been paid to restoration of the capsulolabral complex anatomic insertion onto the glenoid, studies comparing the pressurized contact area and mean interface pressure at the anatomic insertion site between a single-row repair and a double-row labral repair have been uncommon.

Purpose: The purpose of our study was to compare the mean interface pressure and pressurized contact area at the anatomic insertion site of the capsulolabral complex between a single-row repair and a double-row repair technique.

Study Design: Controlled laboratory study.

Methods: Thirty fresh-frozen cadaveric shoulders (mean age, 61 ± 8 years; range, 48-71 years) were used for this study. Two types of repair were performed on each specimen: (1) a single-row repair and (2) a double-row repair. Using pressure-sensitive films, we examined the interface contact area and contact pressure.

Results: The mean interface pressure was greater for the double-row repair technique (0.29 ± 0.04 MPa) when compared with the single-row repair technique (0.21 ± 0.03 MPa) (P = .003). The mean pressurized contact area was also significantly greater for the double-row repair technique (211.8 ± 18.6 mm², 78.4% footprint) compared with the single-row repair technique (106.4 ± 16.8 mm², 39.4% footprint) (P = .001).

Conclusion: The double-row repair has significantly greater mean interface pressure and pressurized contact area at the insertion site of the capsulolabral complex than the single-row repair.

Clinical Relevance: The double-row repair may be advantageous compared with the single-row repair in restoring the native footprint area of the capsulolabral complex.

Background: Recently, some have suggested that the acromioclavicular articulation confers stability to the construct after coracoclavicular ligament reconstruction for acromioclavicular joint separation. Therefore, it has been suggested that distal clavicle excision should not be performed in this context to protect the graft during healing.

Hypothesis: Sectioning the acromioclavicular ligaments would significantly increase in situ forces of a coracoclavicular ligament graft, whereas performing a distal clavicle resection would not further increase in situ graft forces.

Design: Controlled laboratory study.

Methods: A simulated coracoclavicular reconstruction was performed on 5 cadaveric shoulders. Static loads of 80 N and 210 N were applied directly to the clavicle in 5 directions: anterior, anterosuperior, superior, posterosuperior, and posterior. The in situ graft force was measured using a force transducer under 3 testing conditions: (1) intact acromioclavicular ligaments, (2) sectioned acromioclavicular ligaments, and (3) distal clavicle excision.

Results: For both magnitudes of load, in all directions, in situ graft force with intact acromioclavicular ligaments was significantly less than that with sectioned acromioclavicular ligaments (P < .001). Distal clavicle excision did not further increase the in situ graft forces with load applied to the clavicle in an anterior, anterosuperior, or superior direction. However, in situ graft forces were increased with distal clavicle excision when the clavicle was loaded with 210 N in the posterosuperior direction (60.4 ± 6.3 N vs 52.5 ± 7.1 N; P = .048) and tended to be increased with posterior loading of the clavicle (71.8 ± 6.2 N vs 53.1 ± 8.8 N; P = .125).

Conclusion: Intact acromioclavicular ligaments protect the coracoclavicular reconstruction by decreasing the in situ graft force. The slight increase in the in situ graft force only in the posterosuperior and posterior direction after distal clavicle excision suggests only a marginal protective role of the acromioclavicular articulation. Further, the peak graft forces observed represent only a small fraction of the ultimate failure strength of the graft.

Clinical Relevance: Distal clavicle excision can perhaps be safely performed in the context of coracoclavicular ligament reconstruction without subjecting the graft to detrimental in situ force. Although the acromioclavicular articulation serves only a marginal role in protecting the coracoclavicular ligament graft, reconstruction of the acromioclavicular ligaments may serve an important role in decreasing in situ graft force during healing.

Background

There have been few biomechanical studies to clarify which size of a glenoid defect is critical. However, those studies have assumed that the defect occurred anteroinferiorly. Recent studies have reported that the defect is located anteriorly rather than anteroinferiorly. Therefore, the effect of the anterior, not anteroinferior, glenoid defect on shoulder stability needs to be investigated.

Hypothesis

The anterior glenoid defect would have a similar effect on anterior shoulder stability as that of the anteroinferior glenoid defect.

Study Design

Controlled laboratory study.

Methods

Eight fresh-frozen cadaveric shoulders were used (mean age, 74 years). The specimen was attached to a shoulder-testing device with the arm in abduction and external rotation. An osseous defect was created stepwise with a 2-mm increment of the defect width. The stability ratio was used to evaluate joint stability. With a 50-N axial force, the translational force applied to the humeral head was measured by a force transducer.

Results

The stability ratio without a defect (32% ± 6%) significantly decreased after creating a 6-mm defect (17% ± 5%; P = .0001), which was equivalent to 20% of the glenoid length.

Conclusion

An osseous defect at 3 o’clock with a width that was equal to or greater than 20% of the glenoid length significantly decreased anterior stability.

Clinical Relevance

The results suggest that reconstruction of the glenoid concavity might be necessary in shoulders with an anterior glenoid defect of at least 20% of the glenoid length.

Background: There are few biomechanical studies regarding partial-thickness rotator cuff tears and subsequent repair.

Hypothesis: Partial-thickness intra-articular supraspinatus tendon tears increase articular-sided tendon strain as they increase in size. Repair of these tears will return strain to the intact state.

Study Design: Controlled laboratory study.

Methods: Twenty fresh-frozen human cadaveric shoulders were prepared by dissecting to the supraspinatus tendon and leaving the native footprint intact. The tendon footprint was measured with digital calipers and divided into thirds (anterior, middle, and posterior). The middle third was the area where a consistent partial tear was created based on the thickness of the specimens’ particular footprint. Created were 25%, 50%, and 75% tears. Image analysis software and differential variable reluctance transducers strain gauges were used to measure strain. A 100 N load at 1 Hz for 30 cycles was conducted for glenohumeral angles of 45°, 60°, and 90°. This was completed for the intact tendon, 25%, 50%, and 75% tears. Shoulders were then repaired using the in situ fixation method. The in situ method consisted of either a parachute anchor or metal corkscrew anchor. Eight shoulders were examined for load-to-failure testing with the Materials Testing System.

Results: There was a significant difference (P < .05) in rotator cuff strain between the intact rotator cuff tendon and 50% and 75% partial-thickness tears. The cuff strain was returned to the intact state with repair. This was consistent for 3 different glenohumeral abduction angles and for all 3 intra-articular tendon areas. The bursal strain did not have any significant differences between groups.

Conclusion: Articular-sided tendon strain increases consistently across the supraspinatus tendon with greater partial tears. Repair returned strain close to the intact state.

Clinical Relevance: This study may add credence to the clinical practice of repairing intra-articular partial-thickness rotator cuff tears greater than 50%.

Background

Multiple techniques have been reported to treat chronic acromioclavicular joint injuries. However, many have failed to restore native stability, and few have addressed reconstructing the acromioclavicular ligaments.

Hypothesis

An intramedullary free semitendinosus graft reconstruction of the acromioclavicular ligaments will demonstrate joint stability comparable with that of the intact acromioclavicular joint.

Study Design

Controlled laboratory study.

Methods

Six matched pairs of cadaveric specimens with only the acromioclavicular capsule/ligament intact were tested at 10 N and 15 N in the anteroposterior and superoinferior directions under acromioclavicular joint compression loads of 10 N, 20 N, and 30 N. One of each pair randomly underwent reconstruction of the acromioclavicular ligaments with an intramedullary free semitendinosus graft, and the translational testing was repeated. Both the intact and reconstructed specimens then underwent load-to-failure testing via superior clavicle distraction at a rate of 50 mm/min.

Results

The reconstructed specimens reproduced the stability of the intact specimens during all translational and joint compression load trials. Progressively decreased translation was observed for the reconstructed specimens that reached statistical significance (P <.05) in the anteroposterior direction at 10 N of joint compression under 10-N and 15-N translation loads and in the superoinferior direction at 10 N of joint compression and 10-N translational load. With regard to load-to-failure testing, the reconstructed specimens demonstrated significantly lower (P < .05) values for linear stiffness, yield load, ultimate load, and energy absorbed, ranging from 40% to 48% of the values for the intact specimens.

Conclusion

Intramedullary free semitendinosus graft reconstruction of the acromioclavicular ligaments reproduced anteroposterior and superoinferior translational stability and partially reproduced load-to-failure characteristics.

Clinical Relevance

When surgical intervention after higher grade acromioclavicular joint injuries is required, reconstruction of the acromioclavicular ligaments with an intramedullary free semitendinosus graft, in addition to reconstructing the coracoclavicular ligaments, may result in improved stability of the joint complex, improved maintenance of joint reduction, and increased patient satisfaction.

Background

The clinical importance of the biceps reflection pulley (BRP), which stabilizes the long head of the biceps tendon (LHB) as it exits the joint, has been shown. However, there is controversy on the pathomechanism of injury to the BRP. The angular orientation of the LHB relative to its origin and distal course changes with joint positions and may place the BRP at risk for injury.

Purpose

To measure the course of the LHB in common arm positions and to determine the shear and normal (stabilizing) force vectors as well as the excursion of the LHB.

Study Design

Descriptive laboratory study.

Methods

The LHBs of 8 fresh-frozen cadaveric shoulders were marked with arthroscopically injected microbeads and mounted in a custom-built shoulder rig. Data for neutral arm position, forward flexion, and abduction were collected in internal, neutral, and external rotation using biplane fluoroscopy. Bone and LHB position were reconstructed in 3 dimensions.

Results

The shear component of the resulting vector was significantly higher during internal (28.4% ± 18.1%) compared with external rotation (18.9% ± 9.7%; P = .0157) and was highest in neutral arm position with internal rotation (39.2% ± 12.7%) and forward flexion with neutral rotation (36.2% ± 10.7%). The normal force vector, stabilizing the LHB, was significantly higher in abduction (55.2% ± 9.6%) compared with forward flexion (39.1% ± 12.4%; P <.0001) and neutral positions (39.1% ± 11.4%; P <.0001). The LHB excursion was significantly lower for neutral arm positions (0.7 ± 6.0 mm) compared with forward flexion (12.6 ± 8.3 mm; P <.0001) and abduction (12.0 ± 6.5 mm; P <.0001).

Conclusion

Increased shear load at forward flexion with internal or neutral arm rotation and internal rotation at neutral arm position may cause injury to the BRP. Additionally, a sawing mechanism caused by the 12-mm linear excursion combined with a load of the LHB through the BRP during elevation may also lead to lesions.

Clinical Relevance

Knowledge of the pathomechanisms of BRP injury may help in developing specific treatment and rehabilitation strategies as well as tests for physical examination.

Background: Most surgical reconstructions of the separated acromioclavicular joint do not address the injured ligaments and capsule of the acromioclavicular joint.

Purpose: This study was undertaken to compare the biomechanical characteristics of a modified Weaver-Dunn reconstruction and an intramedullary acromioclavicular joint reconstruction that uses a free-tissue graft for reconstruction of both the coracoclavicular and acromioclavicular ligaments.

Study Design: Controlled laboratory study.

Methods: Each pair of 6 matched pairs of cadaveric shoulders was randomly selected for a modified Weaver-Dunn reconstruction on 1 side and the contralateral side was used for free-tissue graft reconstruction of the coracoclavicular and acromioclavicular ligamentous complexes. Anterior-posterior and superior-inferior acromioclavicular joint translation (in millimeters) was measured with acromioclavicular joint compressions of 10, 20, and 30 N, and with translational loads of 10 and 15 N both before and after acromioclavicular joint reconstruction. Load-to-failure testing was then performed for each construct. Repeated-measures analysis of variance (translational testing) and Wilcoxon signed rank test (load-to-failure testing), both with P = .05, were used for statistical analysis.

Results: Mean anterior-posterior and superior-inferior translation of the intramedullary acromioclavicular joint reconstruction was significantly less than that of the modified Weaver-Dunn under all loading conditions (P < .001 and P = .001, respectively), but was not significantly different from that of the intact state (P = .656 and P = .173, respectively). Although the mean ultimate and yield loads and linear stiffness for the intramedullary acromioclavicular reconstruction were greater than that of the modified Weaver-Dunn reconstruction, this did not reach statistical significance (P = .625, P = .625, and P = .625, respectively).

Conclusion: Acromioclavicular joint reconstruction with free-tissue graft for both the coracoclavicular and acromioclavicular ligamentous complexes demonstrates initial stability significantly better than a modified Weaver-Dunn and similar to that of intact specimens.

Clinical Relevance: This acromioclavicular joint reconstruction provides the surgeon with a relatively nondestructive option.