Sagittal plane alignment affects the strength of pin fixation in supracondylar humerus fractures.

ABSTRACT: Closed reduction with percutaneous pin fixation is commonly used to treat pediatric supracondylar humerus fractures. Various pin configurations of varying biomechanical strength have been described. However, to our knowledge, no biomechanical study has focused on pin alignment in the sagittal plane. Our goal was to compare the stability of fixation using 3 different pin constructs: 3 lateral pins diverging in the coronal plane but parallel in the sagittal plane (3LDP), 3 lateral pins diverging in the coronal and sagittal planes (3LDD), and 2 crossed pins (1 medial and 1 lateral).Transverse fractures were made through the olecranon fossa of 48 synthetic humeri, which were then reduced and pinned in the 3LDP, 3LDD, and crossed-pin configurations (16 specimens per group) using 1.6-mm Kirschner wires. The sagittal plane pin spread was significantly greater in the 3LDD group than in the 3LDP group, whereas we found no difference in the coronal plane. Sagittal extension testing was performed from 0° to 20° at 1°/s for 10 cycles using a mechanical torque stand. The torque required to extend the distal fragment 20° from neutral was compared between groups using one-way analysis of variance with multiple comparison post-hoc analysis. P values ≤.05 were considered significant.The 3LDD configuration was more stable than the 3LDP and crossed-pin configurations. The mean torque required to displace the pinned fractures was 5.7 Nm in the 3LDD group versus 4.1 Nm in the 3LDP group and 3.7 Nm in the crossed-pin group (both, P < .01). We found no difference in stability between the 3LDP and crossed-pin groups (P = .45).In a synthetic biomechanical model of supracondylar humerus fracture, sagittal alignment influenced pin construct stability, and greater pin spread in the sagittal plane increased construct stability when using 3 lateral pins. The lateral pin configurations were superior in stability to the crossed-pin configuration.Level of Evidence: Level V.

Using the Remnant Anterior Cruciate Ligament to Improve Knee Stability: Biomechanical Analysis Using a Cadaveric Model.

BACKGROUND: Injured anterior cruciate ligament (ACL) tissue retains proprioceptive nerve fibers, vascularity, and biomechanical properties. For these reasons, remnant ACL tissue is often preserved during the treatment of ACL injuries. PURPOSE: To assess through a cadaveric model whether reorienting and retensioning the residual ACL via an osteotomy improves knee stability after partial ACL tear, with substantial remnant tissue and intact femoral and tibial attachments. STUDY DESIGN: Controlled laboratory study. METHODS: In 8 adult cadaveric knees, we measured anterior tibial translation and rotational laxity at 30° and 90° of flexion with the ACL in its native state and in 3 conditions: partial tear, retensioned, and ACL-deficient. The partial-tear state consisted of a sectioned anteromedial ACL bundle. RESULTS: In the native state, the translation was 10 ± 2.7 mm (mean ± SD) at 30° of flexion and 8.4 ± 3.6 mm at 90° of flexion. Anterior translation of the knees in the partial-tear state (14 ± 2.7 mm at 30° and 12 ± 2.7 mm at 90°) was significantly greater than baseline (P < .001 for both). Translation in the ACL-retensioned state (9.2 ± 1.7 mm at 30° and 7.2 ± 2.1 mm at 90°) was significantly less than in the ACL-deficient state (P < .001 for both), and translation was not significantly different from that of the intact state. For ACL-deficient knees, translation (20 ± 4.3 mm at 30° and 16 ± 4.4 mm at 90°) was significantly greater than all other states (P < .001 for all). Although rotational testing demonstrated the least laxity at 30° and 90° of flexion in the retensioned and intact states and the most laxity in the ACL-deficient state, rotation was not significantly different among any of the experimental states. CONCLUSION: In a cadaveric model of an incomplete ACL tear, a reorienting and retensioning core osteotomy at the tibial insertion of the remnant ACL improved anteroposterior translation of the knee without compromising its rotational laxity. CLINICAL RELEVANCE: The findings of this study support the concept of ACL tissue reorienting and retensioning in the treatment of ACL laxity as an area for future investigation.

Scapholunate Ligament: Comparing the Native Strength of the Ligament to an All-suture Anchor Construct.

Our aim was to compare the tensile strength of the native scapholunate ligament (SLL) with that of an all-suture anchor construct in a cadaveric model. The scaphoid and lunate were isolated, preserving all segments of the SLL. Using a servohydraulic testing machine, we increased the load until peak load-to-failure of the native SLL was reached in nine specimens (mean ± standard deviation, 273 ± 132 N). Using the same specimens, two JuggerKnot 1.4-mm suture anchors (Zimmer Biomet) were placed into the lunate and tensioned through transosseous tunnels in the scaphoid. Sutures were tied over the radial nonarticular aspect of the scaphoid. Load-to-failure testing was repeated. The mean peak load-to-failure for the all-suture anchor constructs was 172 ± 59 N versus 231 ± 117 N for the native group (p = 0.157). This represents approximately 75% of the native ligament strength. (Journal of Surgical Orthopaedic Advances 29(3):169-172, 2020).

Open Biceps Tenodesis: A Biomechanical Comparison of 6 Fixation Techniques.

Tenodesis is used to treat pathology of the long head of the biceps tendon. The authors evaluated the biomechanical properties of 6 techniques for biceps tenodesis fixation. The authors dissected 42 fresh-frozen cadaveric shoulders (mean age, 71±9.8 years; 69% male specimens), leaving the proximal humerus, proximal biceps tendon, and pectoralis major insertion. Specimens were randomized to undergo biceps tenodesis with one of the following: (1) an interference screw; (2) a cortical button; (3) a double-loaded 2.9-mm polyetheretherketone anchor (DL-2.9); (4) a double-loaded 1.9-mm all-suture anchor (DL-1.9); (5) a single-loaded 1.7-mm all-suture anchor (SL-1.7); or (6) soft tissue tenodesis. Specimens then underwent load-to-failure axial traction testing. A generalized linear and latent mixed model with a random-effects term was used to account for specimen pairing. Mean failure loads ranged from 136 N (95% confidence interval, 103-169 N) in the cortical button group to 79 N (95% confidence interval, 58-99 N) in the interference screw group. Failure occurred most often when fixation sutures pulled out of the tendon; however, 7 specimens failed elsewhere. No significant differences in ultimate failure load were found by treatment group. The interference screw group showed significantly more weakness than the cortical button and DL-2.9 groups. Tendon quality and suture parameters are likely more important than the fixation technique in determining failure load. [Orthopedics. 2020;43(2):e102-e108.].

Pullout strength of standard vs. cement-augmented rotator cuff repair anchors in cadaveric bone.

BACKGROUND: We evaluate a novel method of rotator cuff repair that uses arthroscopic equipment to inject bone cement into placed suture anchors. A cadaver model was used to assess the pullout strength of this technique versus anchors without augmentation. METHODS: Six fresh-frozen matched pairs of upper extremities were screened to exclude those with prior operative procedures, fractures, or neoplasms. One side from each pair was randomized to undergo standard anchor fixation with the contralateral side to undergo anchor fixation augmented with bone cement. After anchor fixation, specimens were mounted on a servohydraulic testing system and suture anchors were pulled at 90° to the insertion to simulate the anatomic pull of the rotator cuff. Sutures were pulled at 1 mm/s until failure. FINDINGS: The mean pullout strength was 540 N (95% confidence interval, 389 to 690 N) for augmented anchors and 202 N (95% confidence interval, 100 to 305 N) for standard anchors. The difference in pullout strength was statistically significant (P < 0.05). INTERPRETATION: This study shows superior pullout strength of a novel augmented rotator cuff anchor technique. The described technique, which is achieved by extruding polymethylmethacrylate cement through a cannulated in situ suture anchor with fenestrations, significantly increased the ultimate failure load in cadaveric human humeri. This novel augmented fixation technique was simple and can be implemented with existing instrumentation. In osteoporotic bone, it may substantially reduce the rate of anchor failure.

Pullout strength of cement-augmented and wide-suture transosseous fixation in the greater tuberosity.

BACKGROUND: Obtaining strong fixation in low-density bone is increasingly critical in surgical repair of rotator cuff tears because of the aging population. To evaluate two new methods of improving pullout strength of transosseous rotator cuff repair in low-density bone, we analyzed the effects of 1) using 2-mm suture tape instead of no. 2 suture and 2) augmenting the lateral tunnel with cement. METHODS: Eleven pairs of osteopenic or osteoporotic cadaveric humeri were identified by dual-energy x-ray absorptiometry. One bone tunnel and one suture were placed in the heads of 22 specimens. Five randomly selected pairs were repaired with no. 2 suture; the other six pairs were repaired with 2-mm suture tape. One side of each pair received lateral tunnel cement augmentation. Specimens were tested to suture pullout. Data were fitted to multivariate models that accounted for bone mineral density and other specimen characteristics. FINDINGS: Two specimens were excluded because of knot-slipping during testing. Use of suture tape versus no. 2 suture conferred a 75-N increase (95% CI: 37, 113) in pullout strength (P<0.001). Cement augmentation conferred a 42-N improvement (95% CI: 10, 75; P=0.011). Other significant predictors of pullout strength were age, sex, and bone mineral density. INTERPRETATION: We show two methods of improving the fixation strength of transosseous rotator cuff repairs in low-density bone: using 2-mm suture tape instead of no. 2 suture and augmenting the lateral tunnel with cement. These methods may improve the feasibility of transosseous repairs in an aging patient population.

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.

Gastrocnemius recession: A cadaveric study of surgical safety and effectiveness.

Background and purpose - Many methods of gastrocnemius lengthening have been described, with different surgical challenges, outcomes, and risks to the sural nerve. Our aims were (1) to locate the gastrocnemius muscular-tendinous junction in relation to the mid-length of the fibula (from here on designated the mid-fibula), (2) to compare the dorsiflexion achieved with dorsal recession or ventral recession, and (3) to determine the risk of injury to the sural nerve during gastrocnemius recession. Methods - In 10 pairs of fresh-frozen adult cadaveric lower extremities transected above the knee, we measured dorsiflexion, performed dorsal or ventral gastrocnemius recession at the mid-fibula, and then measured the increase in dorsiflexion and fasciotomy gap. We noted the course of the sural nerve and whether the gastrocnemius muscle provided it with enough muscular coverage to protect it during recession. Results - Dorsal and ventral recession produced statistically (p < 0.05) and clinically significant mean increases in dorsiflexion with extended knee from 12° to 19°, but they were not statistically significantly different from each other in this measure or in fasciotomy gap size. At the mid-fibula, the sural nerve coursed superficially between both heads of the gastrocnemius muscle in 14 of 20 specimens. Sufficient gastrocnemius muscle coverage to protect the sural nerve was provided by the medial head in 18 of 20 specimens and by the lateral head in only 5 of 20 specimens. Interpretation - A ventral gastrocnemius recession proximal to the mid-fibula level poses less risk to the sural nerve than a recession at the mid-fibula. This procedure provides adequate lengthening (1-3 cm) and increased dorsiflexion (compared with baseline), with less risk to the sural nerve than is incurred with recession at the mid-fibular reference line.

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.

Door Opening Affects Operating Room Pressure During Joint Arthroplasty.

Many resources are expended to ensure a sterile operating room environment. Efforts are made to prevent exposure of patients to personnel and to achieve positive room pressure to keep out airborne contaminants. Foot traffic into and out of the operating room during surgery can undermine these efforts. The authors investigated the number and duration of operating room door openings during hip and knee arthroplasty procedures and the effect of the door openings on room pressure. They tested the hypothesis that door openings defeat positive pressure, permitting air flow into the room. Room pressure and door status were monitored electronically during 191 hip and knee arthroplasty procedures. Operating room staff were unaware that data were being collected. The authors evaluated the data with regression analysis to determine whether the number and duration of door openings had an effect on room pressure. Significance was set at P<.05. Doors were open, on average, 9.5 minutes per case. In 77 of 191 cases, positive pressure was defeated, allowing air flow to reverse into the operating room. Total time with the door open significantly affected the minimum pressure recorded in the room (P<.02), but did not significantly affect average room pressure (P=.7). This finding suggested that the loss of positive pressure was a transient event from which the room recovered. The number and duration of door openings showed a significant association with length of surgery. Door openings threaten positive pressure, potentially jeopardizing operating room sterility. The causes of excessive operating room traffic must be evaluated to identify ways to reduce this traffic and the associated risks.

Subject-specific planning of femoroplasty: an experimental verification study.

The risk of osteoporotic hip fractures may be reduced by augmenting susceptible femora with acrylic polymethylmethacrylate (PMMA) bone cement. Grossly filling the proximal femur with PMMA has shown promise, but the augmented bones can suffer from thermal necrosis or cement leakage, among other side effects. We hypothesized that, using subject-specific planning and computer-assisted augmentation, we can minimize cement volume while increasing bone strength and reducing the risk of fracture. We mechanically tested eight pairs of osteoporotic femora, after augmenting one from each pair following patient-specific planning reported earlier, which optimized cement distribution and strength increase. An average of 9.5(±1.7) ml of cement was injected in the augmented set. Augmentation significantly (P<0.05) increased the yield load by 33%, maximum load by 30%, yield energy by 118%, and maximum energy by 94% relative to the non-augmented controls. Also predicted yield loads correlated well (R(2)=0.74) with the experiments and, for augmented specimens, cement profiles were predicted with an average surface error of <2 mm, further validating our simulation techniques. Results of the current study suggest that subject-specific planning of femoroplasty reduces the risk of hip fracture while minimizing the amount of cement required.

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.

Microbial surface contamination after standard operating room cleaning practices following surgical treatment of infection.

At the authors' institution, some joint arthroplasty surgeons require the operating room to be terminally cleaned before using the room after infected cases, in theory to decrease exposure to excessive microbial contamination for the subsequent patient. The authors found no guidance in the literature to support this practice. To test this theory, the authors measured microbial surface contamination from 9 surfaces in operating rooms after standard operating room turnover following 14 infected cases vs 16 noninfected cases. A check was made for an association between organisms isolated intraoperatively from infected surgical patients immediately preceding standard cleaning and organisms isolated from common operating room surfaces. Colony counts were made at 24 and 48 hours, and organisms were identified. No significant difference was noted in colony counts between infected and noninfected cases, and no relationship was found between organisms isolated from infected cases and those from operating room surfaces. Furthermore, the largest colony count from both groups (0.08 cfu/cm(2)) was an order of magnitude less than the recently proposed 5 cfu/cm(2) threshold for surface hygiene in hospitals. This finding indicates that standard operating room turnover results in minimal surface contamination, regardless of the previous case's infection status, and that there is no need for a more extensive terminal cleaning after an infected case.

Survey to determine why people drink raw milk.

BACKGROUND: Fragility fractures associated with osteoporosis extract a large financial and personal toll on society. Pharmaceutical or dietary calcium intake is needed to increase bone mineral density to prevent fragility fractures. Although dairy products are a good source of calcium, patients who are unable to digest lactose tend to avoid them and are put at a greater risk for fracture than the general population. Anecdotal reports suggest that lactose maldigesters, when consuming raw milk, have a dramatic reduction in symptoms relative to pasteurized milk. The mechanism of the reported reduction in symptoms, if true, is unknown. The purpose of the current study was to survey raw milk drinkers to ascertain their health-related motivations for consuming raw milk, especially as they relate to lactose maldigestion. METHODS: An online survey regarding raw milk was completed by 153 of 1527 members of a raw milk-buying community. RESULTS: The primary reason the respondents cited for drinking raw milk was that they believed it was more healthful; 30% reported some gastrointestinal discomfort when drinking pasteurized milk, yet almost all (99%) reported consuming raw milk without discomfort. Despite the reports of gastrointestinal discomfort, only 5% of respondents had been diagnosed as lactose intolerant by a medical professional, and only 1% had been diagnosed as lactose intolerant via the gold-standard hydrogen breath test. CONCLUSIONS: The primary motivation for drinking raw milk is its perceived health value, not its digestibility. Although raw milk appears to be more easily digested than pasteurized milk in our survey sample, the mechanism of digestibility remains unknown.

Biomechanical analysis of posterior intrafocal pin fixation for the pediatric supracondylar humeral fracture.

BACKGROUND: Controversy remains regarding the optimal fixation for displaced pediatric supracondylar humeral fractures. The clinical results of a recently described technique using a posterior intrafocal pin have been good to excellent. The aim of our study was to compare, in a cadaveric model, the stiffness provided by posterior intrafocal pin fixation versus crossed medial and lateral pin fixation and divergent lateral entry pin fixation for the treatment of Gartland-Wilkins type 3 supracondylar humeral fractures. METHODS: In 15 pairs of nonosteoporotic adult cadaver specimens, simulated Gartland-Wilkins type 3 supracondylar fractures were created and stabilized using: (1) the posterior intrafocal pin method; (2) medial and lateral crossed pins; or (3) 2 divergent lateral entry pins. Specimens were then subjected to internal rotation to measure the fixation stiffness of each construct. The effects of treatment and cycle number on torsional stiffness and peak torque were assessed for significance using a linear regression model with random effects to account for specimen pairing. Significance was set at P<0.05. RESULTS: The stiffest fixation was provided by crossed pins (2.4 N m/degree), followed by divergent lateral pins (1.9 N m/degree) and the posterior intrafocal pin (1.9 N m/degree), but none of the differences was statistically significant (P>0.9). Peak torque was not significantly different between fixations, although the trend suggested that crossed pins were strongest (34.6 N m), followed by divergent lateral pins (30.3 N m) and then posterior intrafocal pin fixation (26.1 N m). CONCLUSIONS: Our results suggest that posterior intrafocal pin fixation offers resistance to internal rotation equivalent to that of crossed medial and lateral pins and divergent lateral entry pins. CLINICAL RELEVANCE: The current biomechanical study supports the use of the posterior intrafocal posterior Kirschner pin for rotationally unstable supracondylar fractures because it is not significantly more compliant than standard techniques. LEVEL OF EVIDENCE: Level III.

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.

Use of vertebroplasty to prevent proximal junctional fractures in adult deformity surgery: a biomechanical cadaveric study.

BACKGROUND CONTEXT: Vertebral compression fractures at the proximal junction are common complications of long spinal fusion surgeries that can contribute to the development of proximal junctional kyphosis or proximal junctional failure. To our knowledge, no biomechanical studies have addressed the effect of vertebral augmentation at the proximal junction. PURPOSE: To evaluate the effectiveness of prophylactic vertebroplasty in reducing the incidence of vertebral compression fractures at the proximal junction after a long spinal fusion in a cadaveric spine model. STUDY DESIGN: Biomechanical cadaveric study. METHODS: We divided 18 cadaveric spine specimens into three groups of six spines each: a control group, a group treated with one-level prophylactic vertebroplasty at the upper instrumented vertebra, and a group treated with two-level prophylactic vertebroplasty at the upper instrumented vertebra and the supra-adjacent vertebra. In all spines, the pedicles were instrumented from L5 to T10. Using eccentric axial loading, the specimens were then compressed until failure. Failure was defined as a precipitous decrease in load with increasing compression. The effect of augmentation on load-to-failure was checked using linear regression. The effect of augmentation on incidence of adjacent fractures was checked using logistic regression. Differences at the level of p<.05 were considered significant. KyphX cement introducer was donated by Kyphon, and the pedicle screws were donated by DePuy. RESULTS: Fractures occurred in 12 of 18 specimens: five in the control group, six in the one-level group, and only one in the two-level group; these differences were statistically significant. CONCLUSIONS: Prophylactic vertebroplasty at the upper instrumented level and its supra-adjacent vertebra reduced the incidence of junctional fractures after long posterior spinal instrumentation in this axially loaded cadaveric model. Additional studies are necessary to determine if these results are translatable to clinical practice.

Biomechanical analysis of endobutton versus screw fixation after Lisfranc ligament complex sectioning.

BACKGROUND: Our goal was to compare diastasis after endobutton and screw fixation after Lisfranc ligament complex sectioning. METHODS: Twenty-four (12 pairs) fresh-frozen cadaveric feet were assigned to endobutton or screw fixation and loaded to 343N. Displacement (first-second metatarsal bases) was measured in intact feet and after ligament sectioning (Lisfranc, medial-intermediate cuneiform ligaments), fixation, and 10,000 cycles. RESULTS: The mean change in diastasis for endobutton and screw fixation under initial loading was 1.0mm (95% CI, 0.2-1.9mm) and 0.0mm (95% CI, -0.4 to 0.4mm), respectively (p=0.017). After cyclic loading, diastasis decreased (mean, -0.7mm, 95% CI, -1.2 to -0.1mm) in the endobutton group but was unchanged in the screw group (p=0.035). CONCLUSIONS: Diastasis after endobutton fixation was significantly greater than after screw fixation under initial loading but did not increase further after cyclic loading.