The Effect of Forearm Shortening on Finger Flexion: A Biomechanical Study.

PURPOSE: Surgeons may shorten the forearm for many indications. We quantified the impact of shortening on finger flexion with a cadaver model. METHODS: Ten fresh cadaver proximal forearms were pinned to a static block. We pinned each distal forearm/hand to a block that could unlock, slide, and relock on a mounting track. This block allowed wrist-neutral or 30-degree extension. With the sliding block locked, we removed the central 10 cm of the radius/ulna. We placed sutures in the proximal end of each flexor digitorum profundus (FDP). After pretensioning, we simulated near-maximum baseline FDP muscle-generating force by applying 100 N via a load cell at the proximal sutures. We then anchored the load cell system proximally to set the initial length-tension relationship for simulating near-maximum baseline muscle-generating force. We called subsequent load cell readings the simulated muscle force (SMF) and pressure sensor readings between fingertips and the palm the tip-to-palm force (TPF). We shortened the forearm in 1 cm increments with the distal sliding-locking block. At each increment, we recorded SMF and TPF in the wrist-neutral position. Once a specimen lost measurable TPF, we applied 30 degrees wrist extension until again losing TPF. RESULTS: Incremental forearm shortening was associated with exponential decreases in each FDP's SMF and TPF. In wrist-neutral, 3 cm mean shortening had a loss of 99% and 98% SMF and TPF, respectively. Wrist extension marginally improved SMF and TPF up to 4 cm mean shortening, where both lost 99%. Loss of any fingertip touchdown occurred after a mean shortening of 4.9 cm in wrist-neutral and 5.3 cm in 30 degrees wrist extension. CONCLUSIONS: Mean forearm shortening of 3 or 4 cm had a near-complete loss of FDP SMF and TPF in wrist-neutral/wrist extension, respectively. With ∼5 cm shortening, there was a complete loss of fingertip touchdown. CLINICAL RELEVANCE: Surgeons should consider the influence of forearm shortening on the FDPs and contemplate flexor tendon shortening or alternative reconstructions as indicated.

Biomechanics of subscapularis V-shaped tenotomy compared to standard tenotomy.

BACKGROUND: Subscapularis function is critical after anatomic total shoulder arthroplasty (aTSA). Recently, however, a technique has been described that features a chevron or V-shaped subscapularis tendon cut (VT). This biomechanical study compared repair of the standard tenotomy (ST), made perpendicular to the subscapularis fibers, to repair of the novel VT using cyclic displacement, creep, construct stiffness, and load to failure. METHODS: This biomechanical study used 6 pairs of fresh frozen paired cadaveric shoulder specimens. One specimen per each pair underwent VT, the other ST. Subscapularis tenotomy was performed 1 cm from the insertion onto the lesser tuberosity. For VT, the apex of the V was 3 cm from the lesser tuberosity. After tenotomy, each humerus underwent humeral head arthroplasty. Eight figure-of-8 sutures were used to repair the tenotomy (Ethibond Excel; Ethicon, US LLC). Specimens were cyclically loaded from 2 to 100 N at 45 degrees abduction at a rate of 1 Hz for 3000 cycles. Cyclic displacement, creep, and stiffness and load to failure were measured. RESULTS: Cyclic displacement did not differ significantly between the ST and VT from 1 to 3000 cycles. The difference in displacement between the V-shaped and standard tenotomy at 3000 cycles was 1.57 mm (3.66 ± 1.55 mm vs. 5.1 ± 2.8 mm, P = .31, respectively). At no point was the V-shape tenotomy (VT) >3 mm of average displacement, whereas the standard tenotomy (ST) averaged 3 mm of displacement after 3 cycles. Creep was significantly lower for VT in cycles 1 through 3. For all cycles, stiffness was not significantly different in the VT group compared with the ST group. Load to failure was not statistically significant in the VT compared to the standard tenotomy throughout all cycles (253.2 ± 27.7 N vs. 213.3 ± 76.04 N; P = .25, respectively). The range of load to failure varied from 100 to 301 N for standard tenotomy compared with 216 to 308 N for VT. CONCLUSION: This study showed that VT and ST demonstrated equivalent stiffness, displacement, and load to failure. VT had the benefit of less creep throughout the first 3 cycles, although there was no difference from cycle 4 to 3000. The VT had equivalent biomechanical properties to the ST at time zero, an important first step in our understanding of the technique. The VT technique warrants further clinical study to determine if the technique has clinical benefits over ST following aTSA.

All-Suture Anchor Repair of the Flexor Digitorum Profundus Insertion: A Biomechanical Comparison of 2 Suturing Techniques.

PURPOSE: We compared 2 suturing techniques for reattachment of the flexor digitorum profundus (FDP) via all-suture anchor. METHODS: We used fresh, matched-pair, cadaveric hands. We disarticulated the fingers at the proximal interphalangeal joints, preserving the proximal FDP. We released the FDPs at their distal insertion and placed an all-suture, 1.0-mm anchor at the center of each FDP footprint. Each anchor's sutures were used to reattach each FDP using 1 of 2 techniques: group H (n = 14) via horizontal mattress; group H + K (n = 12) via horizontal mattress with knots thrown and, with each suture tail, 3 proximal, running-locking, Krackow-type passes on the radial and ulnar FDP sides with the suture ends tied together. We excluded 2 specimens from the H + K group because of improper anchor placement. All other fingers in both groups were individually mounted in an MTS machine for FDP loading in the following sequence for 500 cycles each: (1) to 15 N to simulate passive motion forces; (2) to 19 N for short-arc active motion forces; and (3) to 28 N for full active motion forces. Specimens that had not failed during cyclic testing were then loaded to failure. We measured FDP-to-bone gapping via a digital transducer. We defined failure as >3-mm gapping. RESULTS: The H + K group had significantly less gapping during cyclic loading up to 19 N and significantly higher load to failure. The H + K group failed exclusively at the anchor-bone level; the H group failed mostly by suture-tendon pullout. CONCLUSIONS: The H + K group performed significantly better regarding cyclic and load-to-failure testing after FDP reattachment. CLINICAL RELEVANCE: The H + K technique combines the benefits of horizontal-mattress tendon-to-bone apposition and Krackow-tendon locking. It converts the point of failure to the bone level rather than the suture-tendon level.

Comparison of 3 Dynamic External Fixation Devices for Proximal Interphalangeal Joint Dorsal Fracture-Dislocations in a Cadaver Model.

PURPOSE: Several improvised dynamic external fixation devices are used for treating unstable dorsal proximal interphalangeal (PIP) joint fracture-dislocations. We compared the effectiveness of 3 constructs for simulated dorsal PIP joint fracture-dislocations in a cadaver model. METHODS: We tested 30 digits from 10 fresh-frozen, thawed cadaver hands. We aimed to remove the palmar 50% of the base of each digit's middle phalanx (P2), simulating an unstable dorsal PIP joint fracture-dislocation. Each PIP joint was then stabilized via external fixation with either a pins-and-rubber-bands construct, pins-only construct, or tuberculin syringe-pins construct. We allocated 10 digits per fixation group. The finger tendons were secured to a computer-controlled stepper motor-driven linear actuator. Via this mechanism, all PIP joints were taken through 1,400 cycles of flexion-extension. With the PIP joint in neutral extension, we measured the P2 dorsal translation at baseline, after fixator stabilization, and after the motion protocol. RESULTS: The actual mean P2 palmar defect created was 48% of the base. All PIP joints were unstable after creating the defect, with a mean initial P2 dorsal displacement of 3.7 mm. After application of the fixators, all PIP joint dislocations were reduced. The median residual P2 dorsal displacements were 0.0 mm for the pins-rubber bands group, 0.1 mm for the pins-only group, and 0.5 mm for the syringe-pins group. There were no cases of PIP joint redislocation after flexion-extension cycling, and the median dorsal P2 displacements were 0.0 mm for the pins-rubber bands group; 0.0 mm for the pins-only group; and 0.5 mm for the syringe-pins group. CONCLUSIONS: All 3 external fixators restored PIP joint stability following simulated dorsal fracture-dislocation, with all reductions maintained after motion testing. The syringe-pins construct had significantly greater median residual P2 dorsal displacement after the initial reduction and motion testing, which is of unclear clinical importance. CLINICAL RELEVANCE: This study informs surgeon decision-making when considering dynamic external fixator options for dorsal PIP joint fracture-dislocations.

How many anchors to use in arthroscopic Bankart repairs? A biomechanical study of postage-stamp glenoid fractures.

BACKGROUND: Deciding how many anchors to use in a Bankart repair is challenging because of the desire to enhance stability while avoiding a postage-stamp fracture of the glenoid rim. METHODS: This controlled laboratory study investigated load to fracture of the anterior glenoid rim after drilling holes of varying number and diameter and inserting anchors of 2 different types and sizes, with and without perforation of the medial cortex of the glenoid, creating postage-stamp fractures using a metallic humeral head that was compressed against the anterior glenoid rim. A destructive model with a servohydraulic load frame was used to test 46 synthetic scapulae with compressive strength and elastic modulus similar to that of a human glenoid. Load to fracture of the intact glenoid was compared with groups with a varying number of anchor holes of different diameters, with anchors of different sizes and types, and with anchors perforating or not perforating the glenoid medial cortex. The percentage of force to fracture an intact specimen was used to identify relative risk of fracture: low risk >75%, moderate risk 75%-50%, and high risk <50% of intact load. RESULTS: The load to fracture of intact glenoids was 1276 ± 42 N. Loads decreased linearly as the number of holes drilled on the glenoid rim increased. Compared with the 1.6-mm group, the 3.0-mm group had significantly lower glenoid rim strength in specimens with 4, 5, and 7 holes (P = .013, .032, and .045, respectively). All-suture anchors in 1.6-mm holes did not alter the glenoid rim strength, and up to 5 anchors were associated with low risk of fracture. Load to fracture was significantly higher with 3.0-mm rigid core bioabsorbable anchors with 4 anchors (1081 ± 6 N) compared with the 4-hole condition (838 ± 107 N; P = .033). Perforating the glenoid medial cortex with five 1.6-mm anchors significantly weakened the glenoid rim to 58% of intact (P = .012). Perforating the medial cortex weakened the glenoid rim to 52% and 42% (P < .001 for both) of intact in the 3.0-mm 4-anchor and 5-anchor constructions, constituting moderate and high risk of fracture, respectively. CONCLUSION: Up to five 1.6-mm all-suture anchors and four 3.0-mm bioabsorbable rigid-core anchors were associated with low risk of fracture of the glenoid rim. Smaller diameter all-suture anchors best preserved structural integrity of the glenoid rim, whereas tunnel enlargement and perforation of the glenoid medial cortex were associated with moderate or high risk of a postage-stamp fracture.

Impact of radiocapitellar interposition arthroplasty on ulnohumeral joint biomechanics.

BACKGROUND: Radial head excision (RHE) has been shown to increase contact pressures within the ulnohumeral joint. Radiocapitellar interposition arthroplasty (RCIA) with the use of a soft tissue graft is an alternative for the treatment of isolated radiocapitellar arthritis or with failure of radial head replacement. We investigated contact pressures and contact area within the ulnohumeral joint after RHE compared to RCIA with dermal autograft. METHODS: Six fresh-frozen cadaver elbows were tested on a custom dynamic elbow frame. A pressure sensor was inserted into the intact elbow joint, and mean contact pressure, peak contact pressure, contact area, and force within the ulnohumeral joint were recorded at 0°, 30°, 60°, 90°, and 120° of flexion as a valgus load was applied to the elbow. The radial head was then excised and specimens were retested. Finally, a dermal graft matched to the size of the resected radial head was inserted in the radiocapitellar space and the specimens were tested a third time. RESULTS: At 90° of flexion, contact pressure within the ulnohumeral joint was significantly lower with RCIA compared with RHE (110.8 kPa vs 216.8 kPa; P = .013). The mean peak contact pressure was also significantly lower with RCIA compared with RHE at 90° (279.4 vs 626.7 kPa; P = .025). No statistically significant differences were seen in mean contact area or force between the 3 testing conditions at any flexion position. CONCLUSION: RCIA with a dermal graft reduced contact pressures within the ulnohumeral joint compared to RHE at 90° of flexion without a significant change in contact area or contact force.

Effect of reverse shoulder arthroplasty humeral component retroversion on acromial strain.

INTRODUCTION: Acromion and scapular spine stress fractures can be catastrophic complications following reverse shoulder arthroplasty (RSA). A variety of host, implant, and technical factors have been identified that increase the risk of this complication. The glenoid component in particular has been closely evaluated for its impact on rates of stress fractures following RSA. The goal of this biomechanical study is to evaluate if humeral stem version has an impact on acromion and scapular spine strain after RSA. METHODS: Eight cadaveric specimens were tested on a custom dynamic shoulder frame. Commercially available RSA components were implanted with the humeral component inserted in 0° of retroversion. Acromion and scapular spine strain were measured at 0°, 30°, and 60° of abduction using strain rosettes secured to the acromion and scapular spine in the typical locations for Levy type II and type III stress fractures, respectively. The humeral stem was then removed and reimplanted in 30° of retroversion and the measurements were repeated. Student t test was performed to analyze the relationship between humeral stem version and acromion and scapular spine strain at various abduction angles. RESULTS: Strain at the both the acromion and scapular spine were found to have no significant difference at any abduction angle when comparing 0° and 30° version of the humeral stem. With 0° and 30° versions pooled together, there is significantly lower acromion and scapular spine strain at 60° of abduction when compared to 0° of abduction (strain at 0° abduction - strain at 60° abduction: acromion 313.1 µêœª; P = .0409, Scapular spine 304.9 µêœª; P = .0407). There was no significant difference in strain at either location when comparing 0° of abduction to 30° of abduction and when comparing 30° of abduction to 60° of abduction. CONCLUSIONS: This biomechanical study found no significant difference in scapular spine and acromion strain after RSA when comparing variations in humeral stem version. There does appear to be lower strain at both the acromion and scapular spine at 60° of abduction when compared to 0° of abduction regardless of stem version.

Weightbearing after combined medial and lateral plate fixation of AO/OTA 41-C2 bicondylar tibial plateau fractures: a biomechanical study.

BACKGROUND: Combined medial and lateral plate fixation is recommended for complex tibial plateau fractures with medial fragments or no cortical bone contact. Although such fixation is adequate to resist forces during range of motion, it may be insufficient to support immediate postoperative weightbearing. Here, we analyzed displacement, stiffness, and fixation failure during simulated full weightbearing of bicondylar tibial plateau fractures treated with combined medial and lateral locking plate fixation. METHODS: We used 10 fresh-frozen adult human cadaveric tibias and mated femurs. Osteotomies were performed with an oscillating saw and cutting template to simulate an AO Foundation and Orthopaedic Trauma Association (AO/OTA) 41-C2 fracture (simple articular, multifragmentary metaphyseal fracture). Specimens were anatomically reduced and stabilized with combined medial and lateral locking plates (AxSOS, Stryker, Mahwah, NJ). Specimens were loaded axially to simulate 4 weeks of walking in a person weighing 70 kg. The specimens were cyclically loaded from 200 N to a maximum of 2800 N. Then, if no failure, loading continued for 200,000 cycles. We measured displacement of each bone fragment and defined fixation failure as ≥5 mm of displacement. Construct stiffness and load at failure were calculated. Categorical and continuous data were analyzed using Chi-squared and unpaired t-tests, respectively. RESULTS: Mean total displacement values after 10,000 loading cycles were as follows: lateral, 0.4 ± 0.8 mm; proximal medial, 0.3 ± 0.7 mm; distal medial, 0.3 ± 0.6 mm; and central 0.4 ± 0.5 mm. Mean stiffness of the construct was 562 ± 164 N/mm. Fixation failure occurred in 6 of 10 specimens that reached 5 mm of plastic deformation before test completion. In the failure group, the mean load at failure was 2467 ± 532 N, and the mean number of cycles before failure was 53,155. After test completion, the greatest displacement was found at the distal medial fracture site (2.3 ± 1.4 mm) and lateral fracture site (2.2 ± 1.7 mm). CONCLUSIONS: Although combined medial and lateral plate fixation of complex tibial plateau fractures provides adequate stability to allow early range of motion, immediate full weightbearing is not recommended.

Long head of biceps transfer to augment Bankart repair in chronic anterior shoulder instability with and without subcritical bone loss: a biomechanical study.

BACKGROUND: Treating recurrent anterior shoulder instability in participants in collision sports, patients with capsulolabral defects, and patients with bipolar bone loss remains challenging. The study purpose was to investigate the effect of long head of the biceps transfer (LHBT) on load-to-dislocation biomechanics in a repetitive serial shoulder dislocation cadaveric model comparing LHBT with the Latarjet and Bankart procedures-the first-line treatment methods for chronic traumatic anterior shoulder instability with and without anterior glenoid bone loss, respectively. METHODS: In this controlled laboratory study, 8 fresh-frozen cadaveric shoulders with different conditions were dislocated in sequence using a custom test frame. The muscle loading configuration simulated the arm in the apprehension position, and biceps loads of 20 N and 40 N were used for the static glenohumeral position analysis to evaluate the sling effect. Sequential experimental conditions consisted of the intact state, second and third dislocations, chronic instability, Bankart repair, first LHBT, subcritical glenoid bone loss, second LHBT, and Latarjet procedure. RESULTS: The pectoralis major and joint reaction loads to dislocation sequentially decreased with serial dislocations in all specimens, with the lowest value in the subcritical glenoid bone defect condition. In the setting of chronic instability, the pectoralis load to dislocation was significantly higher with the Bankart repair (P = .031) and LHBT (P < .001), at 71% and 85% of the intact value, respectively. Direct comparison of the pectoralis load to dislocation favored LHBT over Bankart repair (P = .015). In the subcritical defect scenario, LHBT did not significantly increase the load to dislocate, and the Latarjet procedure demonstrated a higher load to dislocate than LHBT (P < .001). All 3 surgical procedures tested significantly increased the angle of horizontal abduction at the time of dislocation and restored the glenohumeral position to closer to the intact state. Doubling the biceps load leveraged the sling effect, pulling the humeral head farther posterior-superiorly, but this was not enough to overcome the effect of a 20% subcritical glenoid bone defect. CONCLUSIONS: In a serial dislocation model, LHBT effectively stabilized the glenohumeral joint in a simulated chronic instability scenario, increasing the pectoralis major load-to-dislocate and glenohumeral joint reaction force components at the time of dislocation and restoring relative glenohumeral positioning to close to the intact state. The Latarjet procedure outperformed LHBT in stabilizing shoulders in the 20% subcritical glenoid bone defect condition.

Minimally Invasive Intramedullary Screw Versus Plate Fixation for Proximal Phalanx Fractures: A Biomechanical Study.

PURPOSE: To compare the maximum interfragmentary displacement of short oblique proximal phalanx (P1) fractures fixed with an intramedullary headless compression screw (IMHCS) versus a plate-and-screws construct in a cadaveric model that generates finger motion via the flexor and extensor tendons of the fingers. METHODS: We created a 30° oblique cut in 24 P1s of the index, middle, ring, and little fingers for 3 matched pairs of cadaveric hands. Twelve fractures were stabilized with an IMHCS using an antegrade, dorsal articular margin technique at the P1 base. The 12 matched-pair P1 fractures were stabilized with a radially placed 2.0-mm plate with 2 bicortical nonlocking screws on each side of the fracture. Hands were mounted to a frame allowing a computer-controlled, motor-driven, linear actuator powered movement of fingers via the flexor and extensor tendons. All fingers underwent 2,000 full-flexion and extension cycles. Maximum interfragmentary displacement was continuously measured using a differential variable reluctance transducer. RESULTS: The observed mean displacement differences between IMHCS and plate-and-screws fixation was not statistically significant throughout all time points during the 2,000 cycles. A 2 one-sided test procedure for paired samples confirmed statistical equivalence in fracture displacement between fixation methods at the final 2,000-cycle time point. CONCLUSIONS: The IMHCS provided biomechanical stability equivalent to plate-and-screws for short oblique P1 fractures at the 2,000-cycle mark in this cadaveric model. CLINICAL RELEVANCE: Short oblique P1 fracture fixation with an IMHCS may provide adequate stability to withstand immediate postoperative active range of motion therapy.

The Impact of Dividing the Flexor Tendon Pulleys on Tendon Excursion and Work of Flexion in a Cadaveric Model.

PURPOSE: The A2 and A4 pulleys of the flexor tendon system have traditionally been considered critical components of efficient digital flexion. This dogma has recently been challenged. Using fresh human cadaveric hands and a model to measure force and excursion, we sought to clarify the clinical importance of releasing different pulleys. METHODS: Combinations of A1, A2, and A4 pulleys were released on the index, middle, ring, and little fingers of fresh, cadaveric hands. The excursion was measured as the distance the tendon was pulled by the motor to achieve palm touchdown. The force applied by the motor was constant (25 N); work was derived from the product of force and excursion (distance). The change in excursion and work needed to achieve palm touchdown before and after pulley release was measured. Excursion varies among digits and specimens at baseline; therefore, the percentage change from the intact state was used to compare groups. We compared A2 versus A1, A4 versus A1, A4 versus A2, A1 + A2 versus A2, and A1 + A4 versus A4. RESULTS: Isolated A2 or A4 release had the greatest individual impact on the excursion (4.77% ± 1.52% and 3.88% ± 1.93%, respectively). When A1 was released with A2 (9.90% ± 2.52%), the additional impact on the excursion was significant; however, when A1 was released with A4 (2.63% ± 2.81%), the impact was marginal. No clinically or statistically significant change in the work of flexion was detected. CONCLUSIONS: A1 release was clinically significant when added to A2 release but not when added to A4 release. Sacrifice of the A2 and A4 pulleys resulted in a statistically significant, but clinically negligible, difference in flexor tendon excursion. These data suggest that the A1 pulley should be preserved when other proximal pulley components are likely to be compromised. These data also add further support to the concept that the A2 pulley or the A4 pulley can be released as needed for optimal tenorrhaphy. CLINICAL RELEVANCE: During flexor tendon repair, the length of contiguous pulley release may have more impact on final tendon excursion than which specific pulleys are released.

Biomechanical comparison of zoned-conformity glenoid versus standard glenoid in total shoulder arthroplasty: impact on rotator cuff strain and glenohumeral translation.

BACKGROUND: Current standard total shoulder arthroplasty glenoid implants allow for high levels of glenohumeral mismatch and associated high levels of humeral head translation to improve range of motion and reduce rim stresses on the glenoid. However, high levels of glenohumeral mismatch could also increase glenoid edge loading, eccentric wear, and rotator cuff strain. A zoned-conformity glenoid may be able to reduce the forces on the rotator cuff and glenoid. We compared rotator cuff strain and glenohumeral translation between a standard glenoid (SG) with moderate glenohumeral mismatch and a zoned-conformity glenoid (conforming glenoid [CG]) that limits mismatch. We hypothesized that the CG would have lower levels of strain on the rotator cuff and lower levels of humeral head translation compared with the SG. METHODS: Eight fresh frozen cadaveric shoulders, aged 72 years (range, 67-76 years), were used in this biomechanical study. The specimens were first tested in the intact state. We cycled them 3 times from 0° to 60° of abduction and measured the superiorly-inferiorly and anteriorly-posteriorly directed forces at the joint, compressive forces applied to the glenoid, and humeral head translation. The specimens were then implanted with a standard press-fit humeral component and a polyethylene glenoid with 3 peripherally cemented pegs and a central press-fit peg. Testing was repeated. Finally, the SG was removed, the CG was implanted, and each specimen was tested a third time. RESULTS: The average superiorly directed force at the glenohumeral joint was significantly lower in the intact and CG groups (18.1 ± 18.6 N and 19.8 ± 16.2 N, respectively) than in the SG group (29.3 ± 21.9 N, P = .024). The maximum force directed against the glenoid was also significantly lower in the CG group (87.6 ± 11.7 N) than in the SG (96.0 ± 7.3 N) and intact (98.9 ± 16.5 N) groups (P = .035). No difference was observed in humeral head translation in the anterior-posterior plane from 0° to 60° of abduction (P = .998) or in the superior-inferior plane (P = .999). CONCLUSION: A zoned-conformity glenoid was associated with similar humeral head translation but significantly lower superior forces against the rotator cuff and a significantly lower maximum force against the glenoid compared with an SG implant. These biomechanical findings suggest that a zoned-conformity implant warrants further study in the effort to maintain humeral head translation while reducing rotator cuff and glenoid forces for successful outcomes of total shoulder arthroplasty.

Dermal allograft superior capsule reconstruction biomechanics and kinematics.

PURPOSE: To investigate the effect of a dermal allograft superior capsule reconstruction (SCR) on kinematics and joint pressure biomechanics immediately after simulated superior irreparable rotator cuff tear. METHODS: This controlled laboratory study tested 8 fresh-frozen cadaveric shoulders using a custom test frame. Balanced loading configuration centered the humeral head on the glenoid, and unbalanced load created a force pulling the head toward the acromion. Experimental conditions included the intact rotator cuff, irreparable supraspinatus tear (ISST), and dermal allograft SCR. A digital sensor measured glenohumeral and subacromial contact pressure maps, and a microscribe measured the acromion-humeral distance. RESULTS: Glenohumeral contact pressure of ISST was 175% (295 ± 44 kPa; P = .018) of the intact rotator cuff value (169 ± 10 kPa) at 0° in the balanced condition and 176% (P = .048) of intact at 30°. SCR decreased glenohumeral contact pressure to 110% (185 ± 27 kPa; P = .044) of intact at 0° and to 95% (P = .034) at 30°. Unbalanced ISST contact pressure was 146% (365 ± 23 kPa; P = .009) of intact (250 ± 24 kPa) at 0° and 122% (P = .045) at 60°. SCR decreased contact pressures to 110% (274 ± 21 kPa; P = .039) of intact at 0° and to 89% (P = .003) at 60°. ISST increased superior migration of the humeral head, decreasing the acromion-humeral distance by 3.0 ± 0.6 mm (P = .006) in the unbalanced condition at 0°. SCR increased the acromion-humeral distance to a value similar to that of the intact cuff (P = .003). SCR significantly lowered subacromial pressures in the unbalanced condition. CONCLUSIONS: In an irreparable supraspinatus tear model, the dermal allograft SCR showed competency in stabilizing the glenohumeral joint, decreasing glenohumeral and subacromial contact pressures, and increasing the acromion-humeral distance.

Broach Handle Design Changes Force Distribution in the Femur During Total Hip Arthroplasty.

BACKGROUND: Curved broach handles were developed to overcome limited surgical exposures during total hip arthroplasty. Some authors report increased intraoperative fracture rates during limited exposures. This study evaluates mechanical force ratios transmitted to the bone while broaching with curved vs straight handles. METHODS: An experimental model utilized a 6-axis load cell to measure force distributions produced by 4 different broach handles, each with increasing offset and curvature. Handles were separately impacted and dynamic variables assessed. Handles were then digitized using a high-resolution optical system and a finite element analysis (FEA) was performed to account for trabecular bone and vary the location of mallet impact. Off-axis forces, broaching construct moments, and stress within surrounding bone were computed. RESULTS: Using the experimental model, high-offset handles lost on average 4% more hammering force to the horizontal axis. When the FEA utilized moduli of elasticity to estimate broaching through osteoporotic trabecular bone, horizontally displaced forces (toward cortical bone) were magnified from 4% to a maximum value of 52%. Both the experimental construct and FEA confirmed that larger offset handles increase moment-to-force ratios up to 163%-235%, thus rotating the proximal and distal ends of the broach toward cortical bone. CONCLUSION: Broach handle design is an important determinant of resultant forces transmitted to the broach (and ultimately the bone) during total hip arthroplasty. Unwanted off-axis forces and enhanced rotational dynamics may play a role in intraoperative fractures during femoral canal preparation.

Perilunate Dislocations: Cadaveric Model K-Wire Versus Compression Staple Fixation.

Purpose: Repair of perilunate dislocations requires stabilization of the carpal bones, although there is limited research on the preferred method(s) of stabilization. We compared the biomechanical characteristics of K-wire versus staple fixation of scapholunate (SL) and lunotriquetral (LT) intervals in the wrist. Methods: After creating a grade IV instability model, 20 cadaver wrists were randomized into 2 stabilization model groups using K-wires or surgical staples across the SL and LT intervals. For each ligament, two K-wires or one surgical staple was used to stabilize the proximal carpal row. Each wrist was loaded with 15 N of force and cycled 5,000 times at an angular velocity of 10 rpm. Displacement between bones was measured at different cycles (10, 100, 500, 1,000, 2,000, 3,000, 4,000, and 5,000). A t test was used for comparisons. Results: For the LT K-wire group, the average displacement at 10 cycles was 0.08 mm, and at 5,000 cycles, 0.163 mm. For the SL K-wire group, the average displacement at 10 cycles was 0.025 mm, and at 5,000 cycles, 0.129 mm. For the LT and SL staple fixation models, the average shifts at 10 cycles were 0.029 and 0.063 mm, and at 5,000 cycles, 0.098 and 0.176 mm, respectively. No significant difference between the fixation models was found at any interval. Notably, in each group, a similar number of small outliers with higher amounts of displacement was observed. Conclusions: Either K-wire or surgical staple stabilization is suitable for perilunate dislocations. The surgical method should be selected based on patient-specific circumstances and the provider's judgment. Type of study/level of evidence: Therapeutic III.

Effect of Poly-L-Lactic Acid Mesh Augmentation on Cyclic Gap Formation in Transosseous Patellar Tendon Repair: A Biomechanical Study.

No previous study has investigated poly-L-lactic acid (PLLA) surgical mesh augmentation in the repair of inferior pole patellar tendon rupture. We compared the biomechanical properties of transosseous patellar tendon repair with PLLA surgical mesh augmentation to transosseous repair without augmentation. Ten matched pairs of cadaveric knees were used. Specimens in each pair were randomized to undergo the transosseous technique alone or the transosseous technique augmented with a PLLA surgical mesh. An inferior pole patellar tendon rupture was simulated and the repair procedure was performed. Specimens were cyclically loaded for 500 cycles. Gap formation was measured using two sensors placed medial and lateral to the repair site. After cyclic loading, load to failure was determined by pulling the tendon at a constant rate until a sudden decrease in load occurred. The primary outcome measure was cyclic gap formation at the medial and lateral sensors. Compared with controls, specimens that underwent PLLA mesh-augmented repair had significantly lower medial gap formation at all testing intervals up to 500 cycles (p < 0.05) and significantly lower lateral gap formation at all testing intervals from 10 to 500 cycles (p < 0.05). Transosseous patellar tendon repair augmented with a PLLA woven mesh device provided significantly greater resistance to gap formation compared with transosseous repair alone. These results suggest that PLLA mesh augmentation of the transosseous technique is biomechanically effective for patellar tendon repair.

Effect of Lateral Retinacular Release on Medial Patellofemoral Ligament Reconstruction.

BACKGROUND: When performing a medial patellofemoral ligament (MPFL) reconstruction, surgeons may place the MPFL graft under higher than anatomic tension to minimize the chance of recurrent instability. PURPOSE: To investigate whether a lateral retinacular release (LRR) significantly decreases patellofemoral contact pressures after an overtensioned (OT) MPFL reconstruction. STUDY DESIGN: Controlled laboratory study. METHODS: Mean and peak pressure across the patellofemoral joint at 30°, 45°, and 60° of flexion was assessed in 14 cadaveric knee specimens with intact MPFL, transected MPFL, reconstructed MPFL with graft OT, and OT MPFL with LRR. The Wilcoxon signed rank test was used to determine differences across states, with W and C values calculated when possible. RESULTS: Mean pressure decreased significantly after MPFL transection compared with intact at 30° (456.9 ± 116.8 vs 410.9 ± 109.4 N, P = .006, W < 7) and 45° (404.9 ± 91.7 vs 369.4 ± 85.3 N, P = .005, W < 5) and increased significantly from intact to OT graft at 30° (456.9 ± 116.8 vs 563.0 ± 11.2 N, P = .003, W < 7), 45° (404.9 ± 91.7 vs 481.4 ± 14.8 N, P = .005, W < 5), and 60° (272.9 ± 139.0 vs 367.0 ± 53.7 N, P = .007, W < 3). Peak pressure increased significantly between intact and OT graft at 30° (1364.0 ± 478.2 vs 2094.4 ± 619.8 N, P = .002, W < 9), 45° (1224.7 ± 491.5 vs 1676.7 ± 779.1 N, P = .005, W < 5), and 60° (1117.7 ± 566.8 vs 1604.2 ± 772.9 N, W < 3). In knees with significantly increased mean pressure after overtensioning, mean pressure increased by 23.3% (11/14 knees) at 30°, 18.3% (10/14 knees) at 45°, and 35.0% (10/14 knees) at 60°. Peak pressure increased significantly by 35.3% (30°), 25.2% (45°), and 29.3% (60°). A significant decrease in mean pressure, toward but not to baseline, was observed between the OT and LRR states at 30° (563.0 ± 11.2 vs 501.5 ± 9.3 N, W < 7) and 60° (367.0 ± 53.7 vs 302.0 ± 13.8 N, W < 5) and a decrease in peak pressure at 30° (2094.4 ± 619.8 vs 1886.5 ± 655.3 N; W < 9). CONCLUSION: LRR led to a statistically significant decrease in pressure across the patellofemoral joint in knees that demonstrated increased contact pressures after an OT MPFL graft. CLINICAL RELEVANCE: LRR after an MPFL reconstruction in which the MPFL graft has been OT may help reduce patellofemoral contact pressures at the time of surgery.

Effect of glenosphere lateralization with and without coracoacromial ligament transection on acromial and scapular spine strain in reverse shoulder arthroplasty.

Background: Small changes in deltoid tension and moment arm due to glenosphere lateralization may be associated with an increase in acromion or scapular spine strain in reverse shoulder arthroplasty (RSA), which can lead to stress fracture. The coracoacromial ligament (CAL) may be protective and lower the strain seen on the acromion or scapular spine. This biomechanical study investigated the impact of glenosphere lateralization and CAL integrity on acromion and scapular spine strain after RSA. Methods: Ten cadaveric specimens were tested on a custom dynamic shoulder frame. Acromial and scapular spine strain were measured at 0°, 30°, and 60° of abduction using strain rosettes fixed to the acromion (Levy Type 2) and the scapular spine (Levy Type 3). Specimens were first tested with a standard commercially available RSA implant with zero lateralization and then subsequently with the +3 and +6 lateralizing glenospheres for that implant. The CAL was then cut in each specimen and testing was repeated with the 0, +3, and +6 glenospheres. Maximal strain was recorded at both the acromion and scapular spine and analysis of variance compared strain across various abduction angles and glenospheres with and without CAL transection. Results: In the intact CAL group, maximal strain decreased significantly at the acromion with abduction from 0° to 30° and 0° to 60°, however, at the scapular spine abduction did not significantly impact strain. Maximal strain decreased significantly with increasing abduction from 0 to 30 and 0 to 60 at both the acromion and scapular spine in the cut CAL group. Average strain at the acromion was significantly higher in the cut group (844.7 µÎµ) versus the intact group (580.3 µÎµ), a difference of 31.3% (P = .0493). Average strain at the scapular spine, did not differ in the cut group (725 µÎµ) compared with the intact group (787 µÎµ) (P = .3666). There were no statistically significant differences in acromial or scapular spine strain between various levels of glenosphere lateralization in either the cut or intact state. Conclusion: In this biomechanical study, arm abduction decreased acromial and scapular spine strain following RSA. Cutting the CAL significantly increased strain at the acromion, and did not significantly alter strain at the scapular spine for all angles of abduction, differing from prior literature. Glenosphere lateralization did not have a significant effect on strain at the levels studied regardless of CAL status. Continued study of the complexion relationship between surgical and implant factors on strain following RSA is needed.

Adjustable Cortical Fixation Device for Quadriceps Tendon Repair: A Cadaveric Biomechanical Study.

BACKGROUND: Adjustable cortical fixation devices have demonstrated utility in orthopaedic applications, such as ankle syndesmosis repair. PURPOSE: To assess the cyclic gap formation of a quadriceps tendon repair technique using an adjustable cortical fixation device compared with repair with knotless suture anchors and suture tape, a modification of conventional suture anchor repair. STUDY DESIGN: Controlled laboratory study. METHODS: Eight fresh-frozen matched pairs of cadaveric knees were used. Specimens in each pair were randomized to undergo either modified suture anchor repair (control) or adjustable cortical fixation repair. The control repair was performed as previously described. The experimental repair was performed using 2 No. 2 FiberWire sutures placed into the quadriceps tendon in a running locked Krackow configuration and 2 adjustable loop devices passed through transosseous tunnels. The lagging strands of the devices were tensioned to seat the cortical fixation buttons at the inferior patellar pole and then tied to the free Krackow strands at the superior pole to complete the repair. The mean plastic gap (permanent tendon displacement that did not recover with cyclic extension) and mean maximum gap (peak displacement that occurred with cyclic knee flexion and partially recovered with extension) were evaluated during cyclic loading for 500 cycles of full knee extension to 90° of flexion. RESULTS: At all testing intervals, the mean plastic gap was significantly smaller for the cortical fixation group versus the suture anchor group (P < .02). Similarly, the mean maximum gap was significantly smaller for the cortical fixation specimens at all testing intervals (P < .01). After cyclic loading, the mean maximum gap was significantly smaller in the cortical fixation group (4.80 ± 1.56 mm) versus the suture anchor group (8.47 ± 1.47 mm; P = < .001). The mean plastic gap was also significantly smaller in the cortical fixation versus the suture anchor group (3.25 ± 1.10 mm vs 6.57 ± 1.62 mm, respectively; P = < .001). CONCLUSION: Quadriceps tendon repair using an adjustable cortical fixation device demonstrated superior biomechanical properties in cyclic displacement testing compared with repair using the suture anchor technique. CLINICAL RELEVANCE: These results suggest that an adjustable cortical fixation device is a biomechanically viable alternative for quadriceps tendon repair.

Biomechanical Comparison of Hook Plate vs Headless Compression Screw Fixation of Large Fifth Metatarsal Base Avulsion Fractures.

BACKGROUND: Debate exists on the optimum fixation construct for large avulsion fractures of the fifth metatarsal base. We compared the biomechanical strength of 2 headless compression screws vs a hook plate for fixation of these fractures. METHODS: Large avulsion fractures were simulated on 10 matched pairs of fresh-frozen cadaveric specimens. Specimens were assigned to receive two 2.5-mm headless compression screws or an anatomic fifth metatarsal hook plate, then cyclically loaded through the plantar fascia and metatarsal base. Specimens underwent 100 cycles at 50%, 75%, and 100% physiological load for a total of 300 cycles. RESULTS: The hook plate group demonstrated a significantly higher number of cycles to failure compared with the screw group (270.7 ± 66.0 [range 100-300] cycles vs 178.6 ± 95.7 [range 24-300] cycles, respectively; P = .039). Seven of 10 hook plate specimens remained intact at the maximum 300 cycles compared with 2 of 10 screw specimens. Nine of 10 plate specimens survived at least 1 cycle at 100% physiologic load compared with 5 of 10 screw specimens. CONCLUSION: A hook plate construct was biomechanically superior to a headless compression screw construct for fixation of large avulsion fractures of the fifth metatarsal base. CLINICAL RELEVANCE: Whether using hook plates or headless compression screws, surgeons should consider protecting patient weight-bearing after fixation of fifth metatarsal base large avulsion fracture until bony union has occurred.