Percutaneous Fixation of Unstable Proximal Phalanx Fractures: A Biomechanical Study.

BACKGROUND: Unstable extra-articular proximal phalanx fractures are common injuries to the hand that are often treated by closed reduction and percutaneous pinning. Fracture-induced shortening of the proximal phalanx leads to an extensor lag at the proximal interphalangeal joint. We describe a biomechanical study in cadaver hands to compare the ability of each of three different pin configurations to resist shortening in unstable fractures. METHODS: Seventeen fresh frozen hands were disarticulated at the proximal ends of the metacarpals. The second, third, and fourth proximal phalanges were tested. A 5-mm section of bone was resected from the mid-shaft of proximal phalanx to simulate an unstable fracture. Three techniques were employed and randomized for each finger: transmetacarpophalangeal joint pinning using 1 or 2 Kirschner wires (K-wires) and periarticular cross pinning using 2 K-wires. Compressive axial loads and energy at 1 mm, 2 mm, 3 mm, 4 mm, and 5 mm of subsidence were examined. RESULTS: The forces and energy required to shorten the finger for each amount of subsidence were similar for all 3 pinning techniques and for all 3 finger types. Greater amounts of shortening were found to require larger forces. CONCLUSION: Closed reduction and percutaneous pinning using any of the presented techniques is an adequate method of treatment for unstable proximal phalanx fractures. All of the techniques were equivalent in their ability to resist axial loading, regardless of the complexity of technique, the number of pins used, or finger that was pinned.

Comparison of Exposure in the Kaplan Versus the Kocher Approach in the Treatment of Radial Head Fractures.

BACKGROUND: The aim of this study was to compare the complete visible surface area of the radial head, neck, and coronoid in the Kaplan and Kocher approaches to the lateral elbow. The hypothesis was that the Kaplan approach would afford greater visibility due to the differential anatomy of the intermuscular planes. METHODS: Ten cadavers were dissected with the Kaplan and Kocher approaches, and the visible surface area was measured in situ using a 3-dimensional digitizer. Six measurements were taken for each approach by 2 surgeons, and the mean of these measurements were analyzed. RESULTS: The mean surface area visible with the lateral collateral ligament (LCL) preserved in the Kaplan approach was 616.6 mm2 in comparison with the surface area of 136.2 mm2 visible in the Kocher approach when the LCL was preserved. Using a 2-way analysis of variance, the difference between these 2 approaches was statistically significant. When the LCL complex was incised in the Kocher approach, the average visible surface area of the Kocher approach was 456.1 mm2 and was statistically less than the Kaplan approach. The average surface area of the coronoid visible using a proximally extended Kaplan approach was 197.8 mm2. CONCLUSIONS: The Kaplan approach affords significantly greater visible surface area of the proximal radius than the Kocher approach.

Proximal Humerus Fracture 3-D Modeling.

The objective of this study is to determine the reproducibility and feasibility of using 3-dimensional (3-D) computer simulation of proximal humerus fracture computed tomography (CT) scans for fracture reduction. We hypothesized that anatomic reconstruction with 3-D models would be anatomically accurate and reproducible. Preoperative CT scans of 28 patients with 3- and 4-part (AO classification 11-B1, 11-B2, 11-C1, 11-C2) proximal humerus fractures who were treated by hemiarthroplasty were converted into 3-D computer models. The displaced fractured fragments were anatomically reduced with computer simulation by 2 fellowship-trained shoulder surgeons, and measurements were made of the reconstructed proximal humerus. The measurements of the reconstructed models had very good to excellent interobserver and intraobserver reliability. The reconstructions of these humerus fractures showed interclass correlation coefficients ranging from 0.71 to 0.93 between 1 observer and from 0.82 to 0.98 between 2 different observers. The fracture reduction was judged against normal proximal humerus geometry to determine reduction accuracy. The 3-D modeling techniques used to reconstruct 3- and 4-part proximal humerus fractures were reliable and accurate. This technique of modeling and reconstructing proximal humerus fractures could be used to enhance the preoperative planning of open reduction and internal fixation or hemiarthroplasty for 3- and 4-part proximal humerus fractures.

Enhanced tendon-to-bone repair through adhesive films.

Tendon-to-bone surgical repairs have unacceptably high failure rates, possibly due to their inability to recreate the load transfer mechanisms of the native enthesis. Instead of distributing load across a wide attachment footprint area, surgical repairs concentrate shear stress on a small number of suture anchor points. This motivates development of technologies that distribute shear stresses away from suture anchors and across the enthesis footprint. Here, we present predictions and proof-of-concept experiments showing that mechanically-optimized adhesive films can mimic the natural load transfer mechanisms of the healthy attachment and increase the load tolerance of a repair. Mechanical optimization, based upon a shear lag model corroborated by a finite element analysis, revealed that adhesives with relatively high strength and low stiffness can, theoretically, strengthen tendon-to-bone repairs by over 10-fold. Lap shear testing using tendon and bone planks validated the mechanical models for a range of adhesive stiffnesses and strengths. Ex vivo human supraspinatus repairs of cadaveric tissues using multipartite adhesives showed substantial increase in strength. Results suggest that adhesive-enhanced repair can improve repair strength, and motivate a search for optimal adhesives. STATEMENT OF SIGNIFICANCE: Current surgical techniques for tendon-to-bone repair have unacceptably high failure rates, indicating that the initial repair strength is insufficient to prevent gapping or rupture. In the rotator cuff, repair techniques apply compression over the repair interface to achieve contact healing between tendon and bone, but transfer almost all force in shear across only a few points where sutures puncture the tendon. Therefore, we evaluated the ability of an adhesive film, implanted between tendon and bone, to enhance repair strength and minimize the likelihood of rupture. Mechanical models demonstrated that optimally designed adhesives would improve repair strength by over 10-fold. Experiments using idealized and clinically-relevant repairs validated these models. This work demonstrates an opportunity to dramatically improve tendon-to-bone repair strength using adhesive films with appropriate material properties.

Tart Cherry Prevents Bone Loss through Inhibition of RANKL in TNF-Overexpressing Mice.

Current drugs for the treatment of rheumatoid arthritis-associated bone loss come with concerns about their continued use. Thus, it is necessary to identify natural products with similar effects, but with fewer or no side effects. We determined whether tart cherry (TC) could be used as a supplement to prevent inflammation-mediated bone loss in tumor necrosis factor (TNF)-overexpressing transgenic (TG) mice. TG mice were assigned to a 0%, 5%, or 10% TC diet, with a group receiving infliximab as a positive control. Age-matched wild-type (WT) littermates fed a 0% TC diet were used as a normal control. Mice were monitored by measurement of body weight. Bone health was evaluated via serum biomarkers, microcomputed tomography (µCT), molecular assessments, and mechanical testing. TC prevented TNF-mediated weight loss, while it did not suppress elevated levels of interleukin (IL)-1ß and IL-6. TC also protected bone structure from inflammation-induced bone loss with a reduced ratio of receptor activator of nuclear factor kappa-B ligand (RANKL)/osteoprotegerin (OPG) to a degree comparable to infliximab. Furthermore, unlike with infliximab, TC exhibited a moderate improvement in TNF-mediated decline in bone stiffness. Thus, TC could be used as a prophylactic regimen against future fragility fractures in the context of highly chronic inflammation.

Step-wise medial collateral ligament needle puncturing in extension leads to a safe and predictable reduction in medial compartment pressure during TKA.

PURPOSE: Medial soft tissue release in a varus deformity knee during total knee arthroplasty is essential for accurate balancing of the reconstruction. This study attempts to quantify the effect of sequential needle puncturing of the medial collateral ligament (MCL) using a pressure sensor insert (Verasense by OrthoSensor) and gap measurement under tension. METHODS: Cruciate-retaining arthroplasties were placed in 14 cadaveric knees. The MCL was elongated by step-wise perforation, in five sets of five perforations, with the use of an 18-gauge needle, followed by valgus stress. Following the fifth set of needle perforations, blade perforation was performed on the remaining tense fibers of the MCL. Following each step-wise perforation, corresponding medial compartment pressures and gap measurements under tension were recorded. RESULTS: Sensor measurements correlated closely with step-wise tissue release (R = 0.73, p < 0.0001), and a significant decrease in pressure was found in early needle puncturing (mean 49 N after 5, 83 N after 15, p values < 0.05), although changes diminished at later stages of needle perforation (90 N after 20). Gap measurement demonstrated small gradual changes with early puncturing, but showed significant opening in the later stages of release. There was minimal variation in pressure or gap measurements in flexion versus extension. This finding suggests that MCL needle puncture will not lead to unequal gaps between flexion and extension. There were no cases of MCL over-release after 15 punctures, one case after 20 punctures, and three after blade perforation. CONCLUSION: Needle puncturing of the MCL in extension for up to 15 punctures can be a safe and predictable way to achieve medial opening when balancing a varus knee during TKA as demonstrated in this cadaveric model. Blade perforation should be used with caution to avoid over-release. The needle puncture method can be used by surgeons to achieve reliable reductions in medial compartment pressures, to help achieve a balanced TKA, with minimal risk of over-release.

Effects of Platelet-Rich Plasma and Indomethacin on Biomechanics of Rotator Cuff Repair.

We conducted a study to determine if platelet-rich plasma (PRP) enhances the strength of rotator cuff repair (RCR) and if concomitant use of nonsteroidal anti-inflammatory drugs (NSAIDs) affects PRP efficacy. We also wanted to determine the optimal centrifugation protocol for making PRP from rats. This study used 48 rats, 14 in a centrifugation protocol and 34 in an operative protocol. Six syngeneic rats from the operative group were used as PRP blood donors; the other 28 operative rats underwent bilateral RCRs. The Autologous Conditioned Plasma system (Arthrex) was used to prepare leukocyte-poor PRP. One shoulder was randomized to an intratendinous PRP injection, and the other received normal saline. Each rat was also randomly placed on a postoperative diet, either a regular diet or an indomethacin-enhanced diet. After rats were euthanized at 3 weeks, specimens were dissected to isolate the supraspinatus tendon at its humeral attachment, which was subjected to biomechanical testing. PRP prepared with a protocol of 5 minutes × 1300 revolutions per minute had the highest platelet index. Mean (SD) energy to failure was significantly higher (P = .03) in tendons treated with PRP, 11.7 (7.3) N-mm, than in tendons treated with saline, 8.7 (4.6) N-mm. Both groups (PRP, saline) showed no significant differences between tendons treated with NSAIDs and those not treated with NSAIDs. Intraoperative application of PRP enhances energy to failure after RCR in rats. There were no differences in biomechanical strength with NSAID use and no interactions between PRP and NSAID use.

Reliability of 3-Dimensional Glenoid Component Templating and Correlation to Intraoperative Component Selection.

Although implant-specific intraoperative targeting devices for glenoid sizing exist, a validated method for preoperatively templating glenoid component size in primary total shoulder arthroplasty (TSA) based on digital imaging does not. We conducted a study to determine if 3-dimensional (3-D) digital imaging could be used for preoperative templating of glenoid component size and to compare templated glenoid sizes with implanted glenoid sizes. We created 3-D digital models from 3 glenoid component implant sizes and preoperative scapular computed tomography scans of 24 patients who underwent primary TSA. In study arm 1, surgeons templated the 3-D components using only 2 df (superior-inferior and anterior-posterior planes). In study arm 2, surgeons templated the 3-D components using 6 df (superior-inferior, anterior-posterior, and rotational planes). Overall intraobserver agreement was substantial (0.67) in study arm 1 (P < .001) and moderate (0.58) in study arm 2 (P < .001). In arm 1, overall interobserver agreement was fair (0.36) for trial 1 (P < .001) and fair (0.32) for trial 2 (P < .001). In arm 2, overall interobserver agreement was moderate (0.54) for trial 1 (P < .001) and moderate (0.43) for trial 2 (P < .001). In both arms, surgeons tended to template glenoid components smaller than those implanted intraoperatively, particularly for female patients. Our findings show that 3-D digital models can be consistently and reliably used for preoperative templating of glenoid com-ponent size.

Inter- and intra-observer agreement of femoroacetabular impingement (FAI) parameters comparing plain radiographs and advanced, 3D computed tomographic (CT)-generated hip models in a surgical patient cohort.

PURPOSE: The purpose of our study was to investigate whether advanced, 3D computed tomographic (CT)-generated hip models improves inter-and intra-observer agreement when compared to plain radiographs in identifying femoroacetabular impingement (FAI) morphology. METHODS: Eight consecutive patients who underwent surgery for FAI pathology were selected for this study. Preoperative CT scan image data were used to create high resolution, 3D hip reconstruction models. Four observers (two attending hip surgeons and radiologists) performed a blinded review of preselected radiographs and 3D CT hip models. Alpha and lateral center-edge angle measurements, location of cam lesion and the presence of a "crossover sign" were assessed. Inter- and intra-observer agreement was determined by calculating the intra-class correlation coefficients (ICC) or kappa coefficients to evaluate agreement for categorical variables. RESULTS: The parameter that demonstrated the highest and poorest inter-observer agreement was the presence of a "crossover sign" using 3D CT-generated high resolution hip models (ICC = 0.76, p = 0.00) and anteroposterior pelvis radiography, respectively (ICC = 0.20, p = 0.02). Alpha angle values were significantly higher using plain radiographs when compared to 3D hip reconstruction models (61.1° ± 10.4° versus 55.4° ± 14.4°, p = 0.003). Furthermore, when compared to radiographs, 3D hip reconstruction models demonstrated significantly higher intra-observer agreement (ICC = 0.856 versus 0.405, p = 0.005) when determining the presence of a "crossover sign". CONCLUSIONS: Our findings were suggestive that for most commonly used FAI morphology parameters, CT-generated hip models demonstrated little benefit over plain radiographs in improving inter-observer agreement among providers. LEVEL OF EVIDENCE: III.

Optimal internal fixation of anatomically shaped synthetic bone grafts for massive segmental defects of long bones.

BACKGROUND: Large segmental bone defects following tumor resection, high-energy civilian trauma, and military blast injuries present significant clinical challenges. Tissue engineering strategies using scaffolds are being considered as a treatment, but there is little research into optimal fixation of such scaffolds. METHODS: Twelve fresh-frozen paired cadaveric legs were utilized to simulate a critical sized intercalary defect in the tibia. Poly-ε-caprolactone and hydroxyapatite composite scaffolds 5 cm in length with a geometry representative of the mid-diaphysis of an adult human tibia were fabricated, inserted into a tibial mid-diaphyseal intercalary defect, and fixed with a 14-hole large fragment plate. Optimal screw fixation comparing non-locking and locking screws was tested in axial compression, bending, and torsion in a non-destructive manner. A cyclic torsional test to failure under torque control was then performed. FINDINGS: Biomechanical testing showed no significant difference for bending or axial stiffness with non-locking vs. locking fixation. Torsional stiffness was significantly higher (P=0.002) with the scaffold present for both non-locking and locking compared to the scaffold absent. In testing to failure, angular rotation was greater for the non-locking compared to locking constructs at each torque level up to 40 N-m (P<0.05). The locking constructs survived a significantly higher number of loading cycles before reaching clinical failure at 30 degrees of angular rotation (P<0.02). INTERPRETATION: The presence of the scaffold increased the torsional stiffness of the construct. Locking fixation resulted in a stronger construct with increased cycles to failure compared to non-locking fixation.

A new method for achieving compression in hindfoot arthrodesis.

BACKGROUND: When performing hindfoot arthodeses, one goal of fixation is often to achieve compression across the joint. Traditional lag screws are applied eccentrically, providing compression more on the edge of the fusion. A new technique, using a post in one bone and a lag screw through the post to the other bone, may offer better compression across more of the joint. METHODS: There are three parts to this study comparing a post-and-screw construct to traditional lag screws. Synthetic bone models, representative of the talonavicular joint, were created and assessed for biomechanical measures of compression. Next, the post-and-screw construct was tested in cadavers, under conditions representing early weight bearing after arthrodesis surgery. Finally, 18 patients who had a talonavicular fusion with a post-and-screw construct with one surgeon were compared to the previous 18 patients fixed with traditional screws. RESULTS: In the synthetic bone model, the post-and-screw construct brought the centre of compression closer to the centre of the joint, suggesting compression was less eccentric. Neither traditional screws nor the post-and-screw construct were sufficiently strong to resist early weight bearing forces in cadaver specimens. In the clinical comparison, four patients had a painful nonunion when fixed with traditional screws, compared to none in the post-and-screw construct. CONCLUSIONS: A post-and-screw construct spreads the forces of compression more uniformly across an arthrodesis, even when placed eccentrically. Although not all the biomechanical measures were superior, the post-and-screw construct achieved higher levels of successful fusion in patients. This technology may offer improved outcomes in some clinical scenarios and deserves further study. LEVEL OF EVIDENCE: Level 3.

Influence of Rotator Cuff Tear Size and Repair Technique on the Creation and Management of Dog Ear Deformities in a Transosseous-Equivalent Rotator Cuff Repair Model.

BACKGROUND: Redundancies in the rotator cuff tissue, commonly referred to as "dog ear" deformities, are frequently encountered during rotator cuff repair. Knowledge of how these deformities are created and their impact on rotator cuff footprint restoration is limited. PURPOSE: The goals of this study were to assess the impact of tear size and repair method on the creation and management of dog ear deformities in a human cadaveric model. STUDY DESIGN: Controlled laboratory study. METHODS: Crescent-shaped tears were systematically created in the supraspinatus tendon of 7 cadaveric shoulders with increasing medial to lateral widths (0.5, 1.0, and 1.5 cm). Repair of the 1.5-cm tear was performed on each shoulder with 3 methods in a randomized order: suture bridge, double-row repair with 2-mm fiber tape, and fiber tape with peripheral No. 2 nonabsorbable looped sutures. Resulting dog ear deformities were injected with an acrylic resin mixture, digitized 3-dimensionally (3D), and photographed perpendicular to the footprint with calibration. The volume, height, and width of the rotator cuff tissue not in contact with the greater tuberosity footprint were calculated using the volume injected, 3D reconstructions, and calibrated photographs. Comparisons were made between tear size, dog ear measurement technique, and repair method utilizing 2-way analysis of variance and Student-Newman-Keuls multiple-comparison tests. RESULTS: Utilizing 3D digitized and injection-derived volumes and dimensions, anterior dog ear volume, height, and width were significantly smaller for rotator cuff repair with peripheral looped sutures compared with a suture bridge (P < .05) or double-row repair with 2-mm fiber tape alone (P < .05). Similarly, posterior height and width were significantly smaller for repair with looped peripheral sutures compared with a suture bridge (P < .05). Dog ear volumes and heights trended larger for the 1.5-cm tear, but this was not statistically significant. CONCLUSION: When combined with a standard transosseous-equivalent repair technique, peripheral No. 2 nonabsorbable looped sutures significantly decreased the volume, height, and width of dog ear deformities, better restoring the anatomic footprint of the rotator cuff. CLINICAL RELEVANCE: Dog ear deformities are commonly encountered during rotator cuff repair. Knowledge of a repair technique that reliably decreases their size, and thus increases contact at the anatomic footprint of the rotator cuff, will aid sports medicine surgeons in the management of these deformities.

Comparison of radiographic stress tests for syndesmotic instability of supination-external rotation ankle fractures: a cadaveric study.

OBJECTIVE: According to the classification of Lauge-Hansen, supination-external rotation IV (OTA 44-B) injuries should not have syndesmotic instability; yet, several studies have suggested disruption is present in up to 40% of these injuries based on stress tests. In this study, we examine various stress radiographic parameters in a cadaver model of supination-external rotation IV equivalent injury. We hypothesize that external rotation stress testing and widening of the medial clear space do not always represent syndesmotic instability. Rather, the better predictor of syndesmotic instability will be an increased tibia-fibula clear space with the lateral stress test. METHODS: Eleven fresh frozen human lower limbs were each secured into a custom frame. External rotation stress test was performed by applying an external moment of 7.5 Nm, and lateral stress test was performed by applying 100 N lateral pull at the distal fibula. True mortise radiographs were taken of intact ankles and while performing external rotation and lateral stress tests at each stage of sequentially sectioning the ankle ligaments. The deltoid ligament was sectioned first, then anterior-inferior tibiofibular ligament, posterior-inferior tibiofibular ligament, and interosseous membrane. Tibiofibular clear space and medial clear space were measured on each radiograph. RESULTS: External rotation stress test produced significant medial clear space widening when the deltoid ligaments were sectioned (P < 0.05). Lateral stress test produced no significant widening of the tibiofibular clear space until interosseous membranes were sectioned (P < 0.05). CONCLUSIONS: Lateral stress test with widening of the tibiofibular clear space is the preferred indicator of syndesmotic instability. The external rotation stress is a poor indicator of syndesmotic injury in the setting of deltoid ligament injury.

Rotator cuff repair augmentation with local autogenous bone marrow via humeral cannulation in a rat model.

BACKGROUND: Growth factors have been shown to improve healing after rotator cuff repair. Bone marrow is a potential vehicle for growth factor augmentation, yet methods of delivering marrow to cuff repair sites are still under-researched. We hypothesized that a cannulated humeral implant would deliver local bone marrow and thereby improve healing in a rat model. METHODS: Twenty-eight rats underwent bilateral rotator cuff injury and repair. Each rat acted as its own control, randomized to a cannulated humeral implant in one shoulder and a solid implant in the other. Rats were euthanized at 4 and 8 weeks to create 4 time-treatment cohorts. Tendon healing was evaluated by dimensional measurements, biomechanical testing, and histology. RESULTS: Tendon thickness, all biomechanical measures, and semi-quantitative histologic scores improved over time (P < .05) but not with treatment. The most common site of biomechanical tendon failure was midsubstance in the 8-week cannulated cohort and at the tendon footprint in the other 3 cohorts. Intraluminal bone growth was evident in all cannulated implants. CONCLUSIONS: Humeral cannulation did not quantifiably improve tendon-to-bone healing in a rat model. The diminutive size of implants in rats, however, may have prevented sufficient delivery of local autogenous bone marrow; hence, further study in a larger animal is recommended.

Characterizing bone tunnel placement in medial ulnar collateral ligament reconstruction using patient-specific 3-dimensional computed tomography modeling.

BACKGROUND: Medial ulnar collateral ligament (MUCL) reconstruction is successful in restoring valgus elbow stability, but variability in bone tunnel characteristics exists among surgical techniques. HYPOTHESIS: Tunnel parameters such as diameter, drill angle, and starting location in MUCL reconstruction affect tunnel length and bone bridge size between tunnels. STUDY DESIGN: Descriptive laboratory study. METHODS: Three-dimensional models were created from elbow computed tomography scans of 10 throwing athletes and analyzed using Mimics (Materialise) software. The MUCL reconstructions were simulated on each elbow with 3 techniques: Jobe, humeral docking, and DANE TJ. Humeral central tunnels were modified by diameter, medial-lateral epicondylar starting point, and angle with respect to the humeral axis. Ulnar tunnels were varied by diameter and angle with respect to the ulnar axis. Humeral tunnel length, humeral and ulnar bone bridge sizes, and ulnar tunnel aperture and distance from the articular surface of the olecranon were measured. Comparisons were made using 1- and 2-way analysis of variance and Student-Newman-Keuls multiple comparison tests. RESULTS: Mean central humeral tunnel length varied significantly by starting point and angulation of the tunnel both in sagittal and coronal planes, ranging from 14.2 ± 2.3 mm to 25.5 ± 4.3 mm (P < .05). Mean bone bridge size between humeral exit tunnels ranged from 9.0 ± 2.5 mm to 15.1 ± 3.1 mm, varying by central humeral tunnel orientation and exit tunnel diameter (P < .05). Bone bridge size between ulnar tunnels with the Jobe and docking techniques averaged 6.7 ± 0.9 mm (3.2-mm tunnels) and 6.4 ± 0.8 mm (3.5-mm tunnels), respectively. Angle of ulnar tunnels affected distance from the articular surface with the Jobe and docking techniques (P < .0001) and affected tunnel aperture size with the interference screw technique (P < .0001). CONCLUSION: Humeral and ulnar tunnel angles, starting points, and diameters affect tunnel length, distance from the articular surface, and bone bridge size in MUCL reconstructions. Maximal humeral tunnel length is achieved by starting central or lateral to the midpoint of the epicondyle, angulated 30° to the humeral axis in the sagittal plane and 15° in the coronal plane. A reasonable goal tunnel depth should range from 15 to 20 mm. Ulnar tunnels should be placed on the anterior and posterior aspects of the sublime tubercle, directed away from the joint to minimize the likelihood of breaching the articular cartilage. A bone bridge of 6 to 8 mm between these tunnels can be reasonably achieved. Tunnels with the ulnar interference screw fixation technique should also be directed away from the joint but at an angle more perpendicular than 45° to minimize tunnel aperture size. Regardless of angle of the tunnel drilled for the ulnar interference screw employed in the DANE TJ technique, the tunnel length is sufficient to fully contain a 15-mm screw. CLINICAL RELEVANCE: Computer models can guide MUCL reconstruction technique by indicating tunnel placement for maximizing the bone bridge and tunnel length.

Glenoid implant orientation and cement failure in total shoulder arthroplasty: a finite element analysis.

BACKGROUND: To minimize glenoid implant loosening in total shoulder arthroplasty (TSA), the ideal surgical procedure achieves correction to neutral version, complete implant-bone contact, and bone stock preservation. These goals, however, are not always achievable, and guidelines to prioritize their impact are not well established. The purpose of this study was to investigate how the degree of glenoid correction affects potential cement failure. METHODS: Eight patient-specific computer models were created for 4 TSA scenarios with different permutations of retroversion correction and implant-bone contact. Two bone models were used: a homogeneous cortical bone model and a heterogeneous cortical-trabecular bone model. A 750-N load was simulated, and cement stress was calculated. The risk of cement mantle fracture was reported as the percentage of cement stress exceeding the material endurance limit. RESULTS: Orienting the glenoid implant in retroversion resulted in the highest risk of cement fracture in a homogeneous bone model (P < .05). In the heterogeneous bone model, complete correction resulted in the highest risk of failure (P = .0028). A positive correlation (ρ = 0.901) was found between the risk of cement failure and amount of exposed trabecular bone. CONCLUSIONS: Incorporating trabecular bone into the model changed the effect of implant orientation on cement failure. As exposed trabecular bone increased, the risk of cement fracture increased. This may be due to shifting the load-bearing support underneath the cement from cortical bone to trabecular bone.

A rat model of chondrocyte death after closed intra-articular fracture.

OBJECTIVE: The development of osteoarthritis after intra-articular fractures has been described for decades, although the exact mechanical and cellular changes that occur remain poorly understood. There are several animal models to study this phenomenon, but they are mechanistically different from physiologic fractures in several important ways. This article describes a novel model that recreates the kinematics present in high-energy trauma and intra-articular fractures. METHODS: We designed a "drop tower" for the creation of intercondylar femoral fractures in rats and tested it on cadaveric rats to determine the optimal kinetic parameters. Intra-articular fractures were then created in live rats and the animals were killed at 0, 24, and 72 hours after the fracture. Cartilage samples were obtained for live/dead staining, and the relationships among fracture time, cartilage depth, and cell viability were evaluated. RESULTS: The model reproduced intra-articular fractures very similar to those seen in high-energy trauma, although we required significantly higher energies (3600 mJ) than those reported in other fracture models (40-200 mJ). Cartilage viability decreased with time (68% immediately after the fracture and 46% at 72 hours, P = 0.02) and increased with depth from the articular surface (47% at the surface vs. 66% in the deepest layer, P = 0.001). CONCLUSIONS: This model is a physiologically relevant reliable method for creating intra-articular fractures in rats and can produce meaningful data about the biologic changes occurring in cartilage after injury. Cell viability decreases with time postfracture and with proximity to the articular surface.

Glenoid morphology after reaming in computer-simulated total shoulder arthroplasty.

BACKGROUND: The relationships between reaming parameters for glenoid-implant surface area and bone loss in total shoulder arthroplasty have not been well established. The hypotheses of this study are: (1) for large version corrections, a large reaming depth of 5 mm is not sufficient to obtain complete glenoid implant contact; (2) glenoid bone is removed in a linear proportion with reaming depth; and (3) initial reamer placement has no effect on glenoid bone removal. METHODS: Ten computer models from computed tomography scans of patients with advanced osteoarthritis were created for computer-simulated reaming as performed during total shoulder arthroplasty. Reaming variables studied included reaming depth, reamer placement, and version correction. The resulting reamed glenoid surface area available for implantation and bone volume removed were calculated for each permutation. RESULTS: Reamed surface area significantly increased with larger depths of reaming (P < .0001) and smaller version corrections (P < .0001). Bone volume removed and reaming depth had a strong quadratic relationship (r(2) = 0.999). With off-center reamer placement, volume removed when deviating in the posterior direction was significantly greater than when deviating in the anterior, superior, or inferior direction (P < .05). CONCLUSION: Performing smaller version corrections allows for greater attainable implant-bone surface contact because increasing reaming depth results in small increases in conforming surface area but large losses in glenoid bone stock. Bone volume removed was most sensitive to off-center position errors in the posterior direction.

Glenoid articular conformity affects stress distributions in total shoulder arthroplasty.

BACKGROUND: The stress applied to the glenoid component in total shoulder arthroplasty (TSA) remains an important concern because of the risk of wear and loosening. The purpose of this study was to determine the stress pattern in the glenoid component with 3 different surface designs. METHODS: Computer models of 9 scapulae of patients scheduled for TSA were created from computerized tomography images. Each glenoid was virtually reamed, and 3 different glenoid component designs (conforming, nonconforming, and hybrid) were placed. Using finite element analysis, superior translation of the humeral head was modeled. Maximum stress and shear stress were measured at 3 different locations in the glenoid component: center, transition, and superior regions. RESULTS: All 3 designs showed a similar level of maximum stress at the center and transition regions, while the maximum stress at the superior periphery was significantly higher in the conforming design than in the other 2 designs (P = .0017). The conforming design showed significantly higher shear stress at the superior periphery (P < .0001). DISCUSSION: Stress from periphery loading is higher than from the center and transition region regardless of component design and is highest in the conforming design. The stress at the transition region of the hybrid design was not higher than the other 2 designs. The hybrid design has favorable characteristics based on its low stress at the periphery and greater contact area with the humeral head at the center. LEVEL OF EVIDENCE: Basic Science Study, Biomechanical Computer Simulation Study.

Scaphoid excision and 4-bone arthrodesis versus proximal row carpectomy: a comparison of contact biomechanics.

PURPOSE: We compare scaphoid excision and 4-bone arthrodesis (FBA) with proximal row carpectomy (PRC) in terms of contact pressure, area, and location. METHODS: Six cadaveric forearms underwent simulated FBA with K-wires. We measured pressures in the radiocarpal joint with Fuji contact film after we applied a 200-N load via the wrist tendons with the wrist in neutral, flexion, and extension. We repeated the experiment after excising the lunate and triquetrum, to create a PRC in the same specimens. RESULTS: Contact pressure in the PRC wrist was significantly greater, by 25%, compared with the FBA wrist for all wrist positions. The PRC wrist had a significantly smaller contact area, by 43%, compared with the FBA wrist. In the FBA wrist, lunate contact was more dorsal in flexion but more volar in extension. In the PRC wrist, capitate contact was more dorsal and radial in flexion, whereas the contact was more volar and ulnar in extension. Comparing contact location, FBA contact was significantly more ulnar than PRC contact in wrist flexion. We found no significant difference in contact translation (the distance between the contact locations in the positions of wrist flexion and extension) for the lunate in FBA or the capitate in PRC. CONCLUSIONS: The FBA wrist has significantly lower contact pressure (P < .001), greater contact area (P < .001), and equal contact translation compared with the PRC wrist. These qualities may make FBA less susceptible to degeneration over time. By advancing our understanding of the biomechanics of both wrist procedures, we may better tailor them to the individual patient. CLINICAL RELEVANCE: Current biomechanical evidence is lacking for common motion-preserving procedures for wrist arthritis. Comparing contact pressure, area, and location provides a biomechanical basis of our clinical understanding of these surgeries.