Anchored Transosseous-Equivalent Versus Anchorless Transosseous Rotator Cuff Repair: A Biomechanical Analysis in a Cadaveric Model.

BACKGROUND: The original approach for the repair of torn rotator cuffs involved an open technique with sutures passing through the greater tuberosity and tendon. The development of suture anchors allowed for an all-arthroscopic approach with anchor configurations attempting to re-create a transosseous fixation pattern. Presently, an arthroscopic approach can be combined with a transosseous suture configuration without using anchors. PURPOSE: To evaluate cyclic loading, ultimate load to failure, and the failure mechanisms of transosseous-equivalent (TOE) repair with anchors and anchorless transosseous (AT) repair of rotator cuff tears. STUDY DESIGN: Controlled laboratory study. METHODS: Supraspinatus tears (25 mm) were created in 20 fresh-frozen, human cadaveric shoulders, which were randomized to TOE or AT repair (10 in each group, paired experimental design). Biomechanical testing was performed with an initial preload, cyclic loading, and load to failure. Optical markers were used to monitor gap formation in 3 planes, and the failure mode was recorded. Paired t tests were used to make comparisons of biomechanical parameters between the groups. Multinomial logistic regression was used to compare failure modes between the groups. Significance was set to .05. RESULTS: The TOE group had a significantly higher mean (±SD) ultimate failure load (578.5 ± 123.8 N) than the AT group (468.7 ± 150.9 N) ( P = .034). The TOE group also had a significantly less mean first-cycle excursion (2.97 ± 1.97 mm) than the AT group (4.70 ± 2.04 mm) ( P = .046). There were no significant differences between the groups in cyclic elongation or linear stiffness during cyclic loading. Primary modes of failure were a type 2 tendon tear with medial tendon disruption in the TOE group (7/10) and a type 1 tendon tear with lateral tendon disruption in the AT group (6/10). CONCLUSION: TOE repair resulted in a significantly higher mean failure load compared with AT repair in a cadaveric model. The most common modes of failure were a type 2 tendon tear in the TOE group and a type 1 tendon tear in the AT group. CLINICAL RELEVANCE: A higher mean failure load in TOE versus AT constructs may come at the cost of a less favorable failure mode adjacent to medial anchors at the musculotendinous junction, potentially making revision difficult.

Autologous distal clavicle versus autologous coracoid bone grafts for restoration of anterior-inferior glenoid bone loss: a biomechanical comparison.

BACKGROUND: Treating anterior glenoid bone loss in patients with recurrent shoulder instability is challenging. Coracoid transfer techniques are associated with neurologic complications and neuroanatomic alterations. The purpose of our study was to compare the contact area and pressures of a distal clavicle autograft with a coracoid bone graft for the restoration of anterior glenoid bone loss. We hypothesized that a distal clavicle autograft would be as effective as a coracoid graft. METHODS: In 13 fresh-frozen cadaveric shoulder specimens, we harvested the distal 1.0 cm of each clavicle and the coracoid bone resection required for a Latarjet procedure. A compressive load of 440 N was applied across the glenohumeral joint at 30° and 60° of abduction, as well as 60° of abduction with 90° of external rotation. Pressure-sensitive film was used to determine normal glenohumeral contact area and pressures. In each specimen, we created a vertical, 25% anterior bone defect, reconstructed with distal clavicle (articular surface and undersurface) and coracoid bone grafts, and determined the glenohumeral contact area and pressures. We used analysis of variance for group comparisons and a Tukey post hoc test for individual comparisons (with P <.05 indicating a significant difference). RESULTS: The articular distal clavicle bone graft provided the lowest mean pressure in all testing positions. The coracoid bone graft provided the greatest contact area in all humeral positions, but the difference was not significant. CONCLUSION: An articular distal clavicle bone graft is comparable in glenohumeral contact area and pressures to an optimally placed coracoid bone graft for restoring glenoid bone loss. LEVEL OF EVIDENCE: Basic Science Study; Biomechanics.

Screw insertion in osteoporotic bone: turn-of-the-nut and torque-based techniques provide similar resistance to bone plate slippage.

OBJECTIVES: To measure the resistance to plate slippage provided by a screw inserted to various torsional and rotational endpoints. MATERIALS AND METHODS: A 7-hole, 3.5-mm narrow dynamic compression plate was affixed to an osteoporotic humeral shafts using screws inserted: (1) to 90 degrees after plate contact, (2) to 180 degrees after plate contact, (3) by the 1.4-N·m torque limit method, and (4) by the "2-fingers tight" method. The resistance of the plate to sliding against the bone was measured using a materials testing machine. We checked for an effect of screw insertion method on bone-plate slippage with a general linearized latent and mixed model, controlling for bone mineral density, sex, and specimen clustering. Significance was set at P < 0.05. RESULTS: The force required to slip the plate for 180 degrees of screw rotation was not significantly greater than that of the other insertion groups. CONCLUSIONS: Inserting screws 180 degrees after seating can be expected to yield plate contact to bone similar to that of the "2-fingers tight" standard.

Biomechanical impact of C2 pedicle screw length in an atlantoaxial fusion construct.

BACKGROUND: Posterior, atlantoaxial (AA) fusions of the cervical spine may include either standard (26 mm) or short (16 mm) C2 pedicle screws. This manuscript focused on an in vitro biomechanical comparison of standard versus short C2 pedicle screws to perform posterior C1-C2 AA fusions. METHODS: Twelve human cadaveric spines underwent C1 lateral mass screw and standard C2 pedicle screw (n = 6) versus short C2 pedicle screw (n = 6) fixation. Six additional controls were not instrumented. The peak torque, peak rotational interval, and peak stiffness of the constructs were analyzed to failure levels. RESULTS: The peak torque to construct failure was not statistically significantly different among the control spine (12.2 Nm), short pedicle fixation (15.5 Nm), or the standard pedicle fixation (11.6 Nm), P = 0.79. While the angle at the peak rotation statistically significantly differed between the control specimens (47.7° of relative motion) and the overall instrumented specimens (P < 0.001), the 20.7° of relative rotation in the short C2 pedicle screw specimens was not statistically significantly higher than the 13.7° of relative rotation in the standard C2 pedicle screw specimens (P = 0.39). Similarly, although the average stiffness was statistically significantly lower in control group (0.026 Nm/degree) versus the overall instrumented specimens (P = 0.001), the standard C2 pedicle screws (2.54 Nm/degree) did not differ from the short C2 pedicle screws. CONCLUSIONS: Both standard and short C2 pedicle screws allow for equally rigid fixation of C1 lateral mass-C2 AA fusions. Usage of a short C2 pedicle screw may be an acceptable method of stabilization in carefully selected patient populations.

Can the "turn-of-the-nut" method improve cortical screw fixation?

OBJECTIVES: To determine the angular rotation that maximizes screw tension without sacrificing screw pullout force yet minimizing loss of screw purchase. METHODS: Three pairs of human humeri in each of the 3 groups (osteopenic, osteoporotic, and normal) underwent screw tension and pullout protocols. For screw tension, 3.5-mm screws were tightened into a strain gauge-instrumented plate until screw stripping occurred. Insertion torque, screw tension, and screw rotation were measured. For pullout, 3.5-mm screws were inserted until the head contacted the plate, additionally rotated (90, 180, 270, or 360 degrees), and then pulled out. A generalized linear and latent mixed model was used to check for significant associations (P < 0.05). RESULTS: Mean (95% confidence interval) peak screw tension occurred at 286 degrees (95% confidence interval, 261-311 degrees) beyond screw seating. Screw tension significantly increased at 90-135 degrees but not after 180 degrees. At 270 degrees, 39% of the screws had already reached their peak ability to compress the plate. Peak screw torque lagged behind peak screw tension by 31 ± 50 degrees, and in seeking peak screw torque, a loss of 104 ± 115 N in screw tension resulted. Screw pullout force was greatest at 90 degrees, but it was not significantly different from that of the other angle groups. CONCLUSIONS: Screw rotation at 180 degrees provides screw tension and pullout strength statistically similar to those at greater rotations but without the loss of purchase associated with greater rotations.

Salvaging the pullout strength of stripped screws in osteoporotic bone.

Our goal was to determine whether the pullout strength of stripped screw holes in osteoporotic bone could be increased with readily available materials from the operating room. We inserted 3.5-mm stainless steel nonlocking self-tapping cortical screws bicortically into 5 osteoporotic humeri. Each screw was first stripped by rotating it 1 full turn past maximum torque. In the control group, the screw was pulled out using an MTS machine (858; MTS Inc, Eden Prairie, Minnesota). In the treatment groups, the screw was removed, the hole was augmented with 1 of the 3 materials (stainless steel wire, polysorb suture, or polyethylene terephthalate glycol plastic sheet), and the screws were replaced and then pulled out. The effect of material on pullout strength was checked for significance (P < .05) using a general linearized latent and mixed model (Stata10; StataCorp, College Station, Texas). The mean (95% confidence interval) pullout strength for the unaugmented hole was 138 N (range 88-189), whereas the holes augmented with plastic, suture, or wire had mean pullout strengths of 255 N (range 177-333), 228 N (range 149-308), and 396 N (range 244-548), respectively. Although wire augmentation resulted in pullout strength that was significantly greater than that of the unaugmented screw, it was still below that of the intact construct.

Does maximum torque mean optimal pullout strength of screws?

OBJECTIVES: To determine the relationship between insertion torque and pullout strength of 3.5-mm-diameter cortical screws in cadaveric humeri with different bone mineral densities (BMDs). METHODS: Five pairs of human humeri from each of 3 BMD groups (normal, osteopenic, and osteoporotic) were used. Holes were drilled in each humerus, and maximum insertion torque (T(max)) was measured by tightening a screw until stripping occurred. In the remaining holes, screws were tightened to 50%, 70%, or 90% of the T(max). A servohydraulic testing machine pulled each screw out at 1 mm/s while resulting force and axial displacement were recorded at 10 Hz. The authors checked for an effect of insertion torque (percent T(max)) on pullout strength using a general linearized and latent mixed model (Stata10), controlling for cortical thickness and BMD (T-score). RESULTS: Pullout strength for normal and osteoporotic bone was greatest for screws inserted to 50% T(max) and was significantly greater than that at T(max) but not significantly different from that at 70% or 90% T(max). For osteopenic bone, pullout strength was greatest at 70% peak torque, but it was not significantly different from the pullout strength at the 50% or 90% T(max) levels. CONCLUSIONS: Tightening screws beyond 50% T(max) does not increase pullout strength of the screw and may place bone at risk for damage that might result in loss of fixation. Even after adjusting for bone thickness and density, there is no clear relationship between pullout strength and screw torque.