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Biomechanics of Rotator Cuff Repair
Rotator cuff repairs have the greatest chance of successful healing if their structural integrity is optimized. Biomechanical principles can be assembled into a unified systematic approach to arthroscopic rotator cuff repair that will enable the surgeon to achieve maximal repair strength for all tear configurations.
Indications
1. Persistent pain, unresponsive to nonoperative management that interferes with daily activities
2. Severe night pain that interferes with sleep
3. Weakness, especially the loss of overhead motion
Contraindications
1. Advanced age
2. Very sedentary activity level
3. Poor general health precluding general anesthesia
Mechanisms of Injury
1. Forced abduction, with fall on outstretched hand
2. Direct blow, with fall onto shoulder
Physical Examination
1. Positive impingement sign
2. Weakness with resisted external rotation
3. Assess for subscapularis weakness (positive liftoff; positive Napoleon Test)
4. Subacromial crepitation
Diagnostic Tests
1. Standard radiographs
2. Anteroposterior (AP) view in external rotation
a. AP in internal rotation
b. Axillary
c. Outlet view (to assess acromial morphology)
3. Magnetic resonance imaging (MRI) to assess extent of tear and reveal any associated pathology
4. Arthrogram (if MRI cannot be done)
Special Considerations
The goal is a complete repair, but if complete repair is not possible, consider partial repair to balance the force couples in preference to transfer of an intact functional rotator cuff tendon such as subscapularis.
The general surgical sequences and the biomechanical principles that apply to those sequences are:
1. Tear pattern recognition—two broad categories of tear pattern.
2. Correlation of tear pattern to repair technique.
a. Crescent-shaped tear requires tendon-to-bone repair.
b. U-shaped tear requires margin convergence by side-to-side repair followed by tendon-to-bone repair of the new “converged” margin of the tear.
3. Side-to-side repair (for U-shaped tears)—This requires implementation of the following biomechanical principles:
a. Margin convergence, in which the posterior leaf of the rotator cuff is sutured to the anterior leaf, causing the free margin of the rotator cuff tear to “converge” toward the humeral neck and greater tuberosity (Figs. 12–3A,B). The useful mechanical phenomenon that accompanies margin convergence is that the strain along the free margin of the cuff is significantly reduced by partial repair. For example, if a U-shaped tear is closed in side-to-side fashion for two thirds of the length of the tear, the strain along the “converged” cuff margin is only one sixth the strain that existed prior to the side-to-side closure. This strain reduction is very useful in helping to avoid fixation failure of tendon to bone.
b. Loop security—The loop of suture that secures the leaves of ten don together must be as small as possible to avoid gapping of the repair (Figs. 12–4A,B,C).
c. Knot security—Ideally, the knot should be so secure that it fails by breakage rather than slippage. This is the ultimate in knot optimization. It has been shown experimentally that any basic knot (e.g., stacked half hitches, Duncan loop, buntline hitch) can be converted to a knot that fails by breakage simply by stacking three “reversing half hitches” (half hitches that are sequentially thrown on alternating posts) on top of the basic knot.
d. Balanced force couples—For U-shaped tears, margin convergence achieves a proximal shift of the posterior leaf, which must then be securely anchored to bone to optimize the moment created by the posterior muscle force to balance the anterior moment created by the intact subscapularis (Fig. 12–5). For subscapularis tears, direct repair to bone can achieve balanced force couples if the posterior cuff is intact. In general, there will be a balanced force couple between the anterior and posterior cuff if one can repair at least half of the infraspinatus (for posterior tears) or at least two thirds of the subscapularis (for anterior tears).
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