Sampling
56 fresh frozen shoulders were obtained from unclaimed bodies. Two shoulder specimens were excluded because of gross comminuted scapula fractures. The ages of the specimens ranged from 25 to 46 years old, with a mean of 35+/-11 years old. There were 27 right and 27 left shoulders. There were 10 pairs of female and 17 pairs of male shoulders. There was no gross pathology of the ligaments or bones. None of the shoulders had been previously operated on. The glenohumeral and sternoclavicular joints were disarticulated. The shoulders were dissected free of all skin, muscle and subcutaneous tissues. The clavicles and scapulae were exposed, carefully preserving the acromioclavicular (ACL) and coracoclavicular (CCL) ligaments. No prior sectioning of these ligaments was done to allow accurate simulation of the non-selective nature of clinical ligament injury.
The coracoacromial ligaments were resected at its insertion on the undersurface of the acromion, prior to testing. This removes any confounding effects since the coracoacromial ligaments, often blending in with the inferior acromioclavicular ligaments, may exert an inferior restraining force. No distal clavicle end resection was performed. The specimens were stored at -20 deg. Before the day of the test, each shoulder specimen was thawed overnight at room temperature.
The 54 grossly normal fresh frozen shoulders were tensile tested to failure, using the Instron Machine Model 8846, to compare the structural properties of the i) combined native acromioclavicular and coracoclavicular ligaments, ii) the coracoacromial ligament transfer in modified Weaver-Dunn reconstruction, iii) efmodified Weaver-Dunn reconstruction with the acromioclavicular capsuloligamentous repair, iv) modified Weaver-Dunn reconstruction with the coracoclavicular screw augmentation, v) modified Weaver-Dunn reconstruction with clavicle hook plate augmentation and vi) modified Weaver-Dunn reconstruction with ACJ reconstruction using palmaris-longus tendon graft and mersilene tape augmentation. At a crosshead speed of 50 mm per min, the specimens were tested for superior and anterior displacements. This low crosshead speed used because failure occurs at both a higher load and greater extension if the test is done at high speed, which means that more energy is needed to rupture the specimen at high speed. Stiffening effect of the ligaments could also be minimized at this low rate. Pretensioning was performed at 70 N (physiological load) to reduce the "crimp" effect of the ligaments to straighten the collagen fibres.
The acromioclavicular joint is a true diarthrodial joint formed by the articular surfaces of the outer end of the clavicle and of the acromion. The clavicle and acromion are united by a capsule inserting a few millimeters from the articulating surfaces. This loose capsule is reinforced on the superior and inferior aspect by the powerful acromioclavicular ligament which runs transversely over the joint. The superior component is much better developed and thicker than the inferior acromioclavicular ligament. A resultant force causing ligament failure can be resolved into 3 vectors in the x, y and z axes. The magnitude of a force required to disrupt the abovementioned transverse fibres is the least when applied in a direction perpendicular to the direction of these fibres, as compared to when the force is directed parallel to the direction of these fibres.
The setup of the test rig (Fig. 1), was therefore designed to apply these perpendicular forces to the fibres, in the superior and anterior directions (2 axes). These forces were the most common disruptive forces in injuries. The 3rd axis (distractive force parallel to the direction of the fibres and long axis of the clavicle) subjecting the AC joint to distractive force is not tested since it is uncommon. The anatomical position was defined by aligning the bony articulation of the distal end of the clavicle and the acromion process, with equal tensioning throughout the soft tissue structures. Custom-made clamps were used to mount the clavicle to the crosshead and the scapula to the base of the Instron machine such that a load as perpendicular as possible can be applied. The long axis of the clavicle and the scapular plane were oriented at approximately 90 degrees to one another. To ensure that the coracoclavicular ligament complex is centered under the crosshead, one clamp is placed medially to the CC ligament, while the other is placed in between the CC and AC ligament complexes.
This testing setup assumed that in an ACJ dislocation injury, there was no movement in the sternoclavicular joint (ie, the clavicle and sternum acted as one unit). The values for loads to failure, obtained for this study, were thus the least forces required for ACJ dislocation in the particular direction of interest.
The acromial reference point was defined as the centroid of its surface. With the aid of a proportional divider, the medial boundary of the acromion was determined. The two most anteromedial and posteromedial points of the acromion were then established. A line A, connecting these two points, was drawn and its length measured using a caliper. Line B, with length b, was constructed perpendicularly from line A to the medical concave aspect of the acromion. The medial concave aspect of the acromion, articulating with the lateral end of the clavicle, most closely approximated the arc of a semi-ellipse. The centroid of acromion, coordinate (X, Y), was thus outside the acromion. The midpoint of line A was taken as the mean of all X of the acromion. The mean of all Y for the acromion was described by the formula 4b/(3 × 3.14). If the distance b is zero, then the acromion was in total contact with the lateral end of the clavicle. [See Additional file 1]
The distal clavicular reference point was defined as the point on the clavicle in contact with the acromial reference point in the intact, unloaded joint. The joint separation, in response to a known applied load, was determined along 2 axes. The posterior-anterior displacement was defined as the distance between the point of maximum anterior displacement of the clavicle reference point and the neutral position of the clavicle reference point (corresponding to the application of the 100-N force anteriorly). The superior displacement was defined as the distance between the point of maximum superior displacement of the clavicle reference point and the neutral position of the clavicle reference point.
Increasing load was then applied to each specimen until the testing endpoint was achieved, that is complete tear of ACJ and ligament, complete failure of ligament reconstruction or complete failure of reconstruction-augmentation construct and specimen failure. Superior displacement in the coronal plane and anterior displacement in the sagittal plane were determined by measuring joint separation as the clavicle was loaded in the superior and anterior directions respectively. There was no movement between the clamps and specimens during testing. The movement from the AC joint was equal to the displacement of the load cell and recorded simultaneously by the Instron machine software, as the loads were being generated. Parallel reference indicators (linear frames with accuracy to 0.1 cm) attached to either side of the load cell also allows measurement of separation, with error of +/- 6%. The respective failure loads, displacement at failure, stiffness and modes of failure were recorded. When "failure" status was reached, the load-cell returned the clavicle to its original pre-tensioned resting position, with respect to the acromion, as preset in the software program. Unless a fracture or deformation occurred, the same scapula and clavicle was used for each of the subsequent reconstructions.
The order of testing sequence was not randomized and executed in the following manner:
Testing Sequence
(1) Superior Loading.
Native Lig → WD → End Point (38 Specimens).
→ WD + ACJ → End Point (9 Specimens).
→ WD + CP → End Point (10 Specimens).
→ WD + BS → End Point (10 Specimens).
→ WD + PL-MT → End Point (9 Specimens).
(2) Posterior-Anterior Loading.
Native Lig → WD → End Point (16 Specimens).
→ WD + ACJ → End Point (8 Specimens).
→ WD + PL-MT → End Point (8 Specimens).
("→ " implies tested to failure)
Reconstruction and Augmentation Techniques
• Modified Weaver-Dunn reconstruction.
The modified Weaver-Dunn reconstruction (Fig 2) was performed by dividing the coracoacromial ligament at its acromial insertion. The freed acromial end of the coracoacromial ligament was anchored with whipstick sutures using No.2 Ethibond sutures (Johnson and Johnson). Prior templating of the future 3.5 mm drill-hole sites was made with the clavicle hook plate sitting on the superior aspect of the clavicle. The stump of the coracoacromial ligament was drawn into one of the middle drill-holes through the inferior cortex and out of the superior cortex of the clavicle. The sutures were then tied around the anterior half of the clavicle. Repair of the acromioclavicular capsuloligamentous complex was performed using Bunnell-type weave with No. 2 Ethibond suture. The distal ends of the clavicles were not resected to allow for repair of the acromioclavicular capsuloligamentous complex and optimal plate sitting on the clavicle.
• ACJ capsuloligamentous repair.
The acromioclavicular capsuloligamentous complex using a Bunnell-type weave with No 2 Ethibond sutures.
• Clavicle hook plate augmentation.
The acromioclavicular joint was reduced under vision. The clavicle hook plates, (Fig 3), with 6 or 8 holes, are precontoured in left and right plates. They are available in commercially pure titanium and stainless steel. The hook of the plate (Synthes) with a 15 mm or 18 mm hook depth was first passed under the acromion, then on the superior aspect of the clavicle. Finally, 3.5 mm cortical screws were placed in the medial and anterolateral screw holes. The coracoacromial ligament graft can be tunneled into one of the middle screw holes of the plate. The plate with 18 mm hook depth is used instead if there is difficulty lowering the plate shaft onto the clavicle.
Its use is especially advantageous in situations where concomitant coracoid process fracture precludes the use of bioabsorbable tape slings or coracoclavicular screw fixation.
• Coracoclavicular screw augmentation (Fig 4).
A modification of the method described originally by Bosworth was performed. [2] The AO cortical screw (Synthes) was positioned starting from the posterior part of the clavicle 4 cm from its lateral end and passing forward and downward to be inserted into the base of the coracoid process. A 4.5 mm hole was first drilled in the clavicle and then a 3.2 mm drill, passing through this hole, advanced into the base of the coracoid. A 4.5 mm AO screw of adequate length, with a large washer, was now inserted through the hole and screwed into the coracoid until the clavicle was compressed onto the coracoid. Bicortical fixation was achieved, with the inferior cortex being breached by 2 threads of the screw.
• Palmaris Longus tendon – Mersilene tape augmentation (Fig 5).
Palmaris Longus tendon grafts were prepared after being harvested from the volar aspect of cadaveric forearms via two 1-cm transverse mid-axial incisions spaced about 10 cm apart. Prior to testing, a tendon graft was then passed through the 3.2 mm holes, each drilled at the distal end of the clavicle and at the acromion, 1 cm away from the acromioclavicular joint with the ends secured in a pulvertaft fashion, using No.2 Ethibond sutures, This reconstruction was reinforced with a Mersilene tape which was passed beneath the coracoid process, swung and tied on the superior aspect of the distal third of the clavicle.
Load-displacement values were analyzed for each test to determine structural properties, that is, load to failure (in newtons), stiffness (in newtons per millimeter) and displacement at failure load (in millimeters). The load to failure and displacement at failure represents the load and point at which the native ligaments fail completely. These results were recorded directly from the computer. The linear stiffness was calculated by determining the slope of the line fit to the linear portion of the load-elongation curve.
Load-displacement values were plotted simultaneously. These results for the clavicle hook plate more accurately reflect the load at which the distal clavicle end fractures or acromion fractures or when the hook dislodges from the inferior surface of the acromion.
Statistical Analysis
A one-way analysis of variance was used for multiple comparisons amongst the 5 groups, with respect to load to failure, displacement at failure and tensile stiffness. (S-PLUS statistical software 2005). The Student's paired t-test was used only for comparison between sequential testing of native ligaments and WD reconstruction in the same specimen. Unpaired specimens were analyzed using Student's unpaired t-test. A p-value of 0.05 was used to denote the level of significance.
