The effect of two different trochanteric nail lag-screw designs on fixation stability of four-part intertrochanteric fractures: a clinical and biomechanical study.

OBJECTIVES: To compare lag-screw sliding characteristics and fixation stability of two cephalomedullary nails (CMN) with different lag-screw designs (solid and telescopic), we conducted a biomechanical study and an analysis of clinical results. METHODS: Six pairs of cadaver femurs with simulated intertrochanteric fractures were randomly assigned to one of two CMN fixations. Femur constructs were statically then cyclically loaded on an MTS machine. Lag-screw sliding and inferior and lateral femoral head displacements were measured, following which failure strength of the construct was determined. Forty-five patients with intertrochanteric fractures treated with these CMN were identified. Medical records and radiographs were reviewed and analyzed using Fisher's exact test and Student's t test to determine lag-screw sliding. RESULTS: No difference was seen with cycling in inferior femoral head displacement between the two screw designs. The solid screw had an average inferior head displacement of 1.75 mm compared with 1.59 mm for the telescoping screw (p = 0.772). The solid lag screws slid an average of 2.79 mm lateral from the nail, whereas the telescoping screws slid an average of 0.27 mm (p = 0.003). In our clinical review, the average lateral sliding of the telescoping screw was 0.5 mm and of the solid screw was 3.7 mm (p < 0.001). Despite differences in lateral sliding, there were no reoperations for prominent or painful hardware in either group. CONCLUSIONS: Both designs are acceptable devices for stabilization of intertrochanteric fractures. Clinical and biomechanical data demonstrate greater lateral sliding in the solid lag-screw group, making for greater potential for lateral-sided hip pain in CMNs with solid lag screws as opposed to telescoping lag screws.

Partial subscapularis release for total shoulder arthroplasty: a biomechanical comparison of two techniques.

BACKGROUND: Glenohumeral joint exposure during total shoulder arthroplasty (TSA) is obtained by releasing the subscapularis (SSC) with either an osteotomy or a tenotomy. Recently, concerns regarding SSC dysfunction after TSA have been raised. In order to avoid this complication, alternative surgical approaches that release the inferior 50% or 10% of the tendon have been described. While a 10% release of the SSC would theoretically lower the likelihood of postoperative SSC dysfunction, releasing 50% would provide greater surgical exposure but possibly have a weaker SS attachment. Therefore, we sought to compare the SSC attachment strengths of these two techniques. MATERIALS AND METHODS: Each of eight matched pairs of cadaveric shoulders were tested. The inferior 10% of the SSC tendon was released on one side. On the contralateral side, the inferior 50% of the SSC was released and then repaired with a 5.5 mm suture anchor. The specimens were then mechanically tested to failure. RESULTS: The load to failure for the 10% release specimens was 682 ± 153 N and 493 ± 212 N for the 50% release specimens (p = 0.036). Failures in both groups occurred mainly at the musculotendonous junction. DISCUSSION: The SSC humeral attachment strength after releasing the inferior 10% was 30% greater than the 50% re- lease with repair. Thus, although releasing the inferior 50% of the SSC tendon may provide greater surgical exposure, maintaining the SSC with minimal release may be preferable in decreasing the rate of post TSA SSC dysfunction.

A laboratory comparison of a new arthroscopic transosseous rotator cuff repair to a double row transosseous equivalent rotator cuff repair using suture anchors.

BACKGROUND: Because current instrumentation makes it possible to perform an arthroscopic transosseous rotator cuff repair, we performed a biomechanical comparison of a double-row transosseous equivalent rotator cuff repair using suture anchors to an arthroscopic, transosseous rotator cuff repair to determine if they provided similar fixation stability. METHODS: Six pairs of shoulders were used. One of each pair had a standard double row, transosseous equivalent arthroscopic rotator cuff repair using a suture-bridge technique with suture anchors, and the other had an arthroscopic transosseous repair using an Xbox technique. The repairs were cycled at 150 N for 10,000 cycles with movement of the lateral cuff edge recorded and then tested to failure. RESULTS: The total cuff edge displacement at 10,000 cycles in the anchor group (transosseous equivalent repair) was 7.9 mm and 6.3 mm for the bone tunnel group (transosseous repair); these were not significantly different (p=0.19). The anchor group failed at an average of 309 N and the bone tunnel group at an average of 339 N (p=0.22). DISCUSSION: Biomechanical testing suggests that arthroscopic, transosseous rotator cuff repair using a Xbox suture configuration is similar in strength and stability to an arthroscopic transosseous equivalent suture-bridge repair. Both techniques demonstrated difficulty in maintaining the lateral position of the tendon.

Midfoot fusion: a biomechanical comparison of plantar planting vs intramedullary screws.

BACKGROUND: Numerous reconstructive techniques for midfoot collapse secondary to Charcot neuroarthropathy have been described, but few have been studied biomechanically. The purpose of this study was to biomechanically compare 2 of the most common techniques. METHODS: Seven paired below-knee specimens were amputated through the talonavicular and calcaneocuboid joints. The nonligamentous soft tissue was stripped proximal to the metatarsal heads and disarticulated through the tarsometatarsal (TMT) joints. For each paired specimen, the TMT joints were fused by plantar plating or intramedullary screw fixation for the contralateral side. The specimens were mounted, loaded, and cycled, and fixation stiffness was determined. Load versus displacement graphs were used to calculate overall construct stiffness, and data were analyzed by Student t tests. RESULTS: There was no failure of hardware. All failures were at the bone-implant interface. Failure was either by screw pull-out, bone fracture, or a combination of the two. There were no notable differences between the 2 fixation techniques with respect to stiffness or loads to failure. There was a trend toward a stiffer first TMT construct using the plantar plating method. Five of the 7 screw fixations failed by pullout of the base of the first metatarsal and the other 2 by pullout of screws from all MT bases. Seven of the 7 plantar plate fixations failed by separation of the fifth to third MT bases originating at the fifth, and 3 showed fracture of the fifth metatarsal base. CONCLUSIONS: There was no notable biomechanical difference between the 2 techniques. There was a trend toward a stiffer construct at the first TMT with plantar plating. CLINICAL RELEVANCE: This study biomechanically analyzes two common Charcot midfoot reconstruction techniques and highlights the need for further study of both techniques and combinations of these techniques.

Effects of suture tension on the footprint of rotator cuff repairs--technical note.

The footprint is thought to be an important fact in rotator cuff repairs and has been used to compare various cuff fixation techniques. The following experiment used two different measurement sensors to evaluate the footprint as it is affected by suture tensions in a transosseous equivalent suture bridge repair. It was found that suture tension has a direct effect on footprint contact area and pressure and thus could affect healing and fixation stability and should be characterized in any study of comparative fixation techniques.

Case report: analysis of an ultra-high-molecular-weight polyethylene acetabular cup retrieval at 41 years.

A polyethylene cup explanted after 41 years was examined using several analytical techniques to determine whether there was a material cause for the extremely low wear observed. Neither the amount of polyethylene oxidation nor crystallinity appeared to be a factor.

Sliding of two lag screw designs in a highly comminuted fracture model.

A fracture construct, representing a worst-case model of a comminuted intertrochanteric fracture, was created in order to compare the fixation stability of two different cephalomedullary nails: one where the lag screw can telescope within itself to achieve displacement of the head-neck fragment, and the other where the solid lag screw slides only. After nail fixation, the models were loaded and then cycled, and positions of the head-neck fragment and lag screw were determined. Both nails similarly acted to limit motion of the head-neck fragment by the sliding of their lag screws, causing impingement of the fragment against the nail. Fragment movement was achieved with significantly less force with the telescoping lag screws, which also showed no final lateral projection from the nail. This was in contrast to the solid lag screws that demonstrated lateral projection in all cases.

Biomechanical comparison of translaminar screw versus pedicle screw supplementation of anterior femoral ring allografts in one-level lumbar spine fusion.

Pedicle screws (PS) can provide initial stabilization of anterior interbody femoral ring allograft (FRA) lumbar constructs. Translaminar screws (TLS) have also been advocated for this procedure. The objective of this study was to use an in vitro human cadaveric model to compare the stability of one-level anterior interbody lumbar constructs stabilized with PS and those stabilized with TLS. Five human cadaveric spinal motion segments (L4-S2) were biomechanically evaluated in the intact condition and using the follow- ing methods of stabilization: anterior interbody fusion with FRA, anterior FRA supplemented with PS, and anterior FRA supplemented with TLS. Stability was determined for each construct by measuring construct displacement as a function of applied load under the following conditions: compression, flexion, extension, lateral bending to each side, and axial torsion. There were no statistically significant differences in construct stability between FRA supplemented with PS and FRA supplemented with TLS under any of the loading conditions. In selected cases, supplementation of anterior femoral ring allograft with translaminar screws is a viable alternative to supplementation with pedicle screws.

Helical blade vs telescoping lag screw for intertrochanteric fracture fixation.

The purpose of this study was to compare fixation stability and lag screw sliding characteristics between 2 different hip-nail lag screw designs, a telescoping screwbarrel and a solid helical blade. Simulated, unstable, 4-part intertrochanteric hip fractures were created in 6 pairs of cadaveric femurs. Each nail type was randomly assigned within each femur pair. Lag screw sliding and inferior and lateral head displacements were measured following an applied static load of 750 N. Measurements were obtained before, during, and after cyclical loading with 750 N for 105 cycles. Ultimate failure strength was determined. After considering inferior head displacements, no significant differences between the 2 screw designs were found. Mean head displacement for the helical screw was 2.18 mm, compared with 1.87 mm for the telescoping screw (P = .731). A significant difference in the amount of lateral movement of the lag screws was found, however. The helical lag screws had mean lateral sliding of 2.68 mm, compared with 0.25 mm for the telescoping screws (P = .007). Neither of the lag screw constructs failed by screw cutout from the head. Both screw designs provide similar fixation strength for stabilization of 4-part intertrochanteric fractures. Both the telescoping lag screw and the helical blade facilitate fracture collapse, but the telescoping lag screw also minimizes lateral projection of the screw from the nail. This advantage may help minimize postoperative lateral soft-tissue impingement.

Acoustic emission studies of posterior stabilized and cruciate retaining knee arthroplasties.

Different acoustic frequencies have been used to diagnose progression of osteoarthritis, gross pathology, and wear in knee prostheses. It is possible that detailed analysis of higher frequencies could detect and quantify the smaller geometric changes (asperities) that develop in articular prosthetic wear. In this study we evaluated the feasibility of using ultrasonic emission to determine total knee arthroplasty (TKA) type and time from implantation using a simple, handheld measurement system. We examined the ultrasound emission generated by similar designs of posterior stabilized (PS) and cruciate retaining (CR) total knee prostheses and native knees of 58 patients and 10 controls. The subjects were asked to sit, rise, sit again, and take five steps while recording the acoustic data from both knees. Acoustic emission analysis examined frequency distributions and power spectrums of the recorded signals, and their relations to prosthesis type and time from implantation. We screened 44 CR and 48 PS TKAs, as well as 24 native knees. Analysis of this data suggested a possibility of differentiating between type of implants, and a relation to time since implantation. Our data suggest that we might be able to assess the status and time from implantation of a TKA by acoustic emission signals. Further in vitro analysis of the relationship of wear to ultrasonic emission data are needed for accurate quantification of arthroplasty wear. A simple, in-office screening tool for TKA patients could indicate which patients require closer follow-up and monitoring due to risk of potential problems.

Failure of the tibial insert in a rotating hinge total knee arthroplasty.

There has been a steady increase in the number of revision total knee arthroplasties being performed in the United States. Hinge knee prostheses are used predominantly in complex primary or revision arthroplasties, often as salvage procedures. Significant improvement of the articulation between the rotating hinge mechanism and tibial component has decreased the stresses that contributed to earlier failures in previous generation designs. Two cases of fracture of the tibial metal post in the rotating hinge of a revision total knee arthroplasty were evaluated and treated at our institution within a 1-month period. We present our experience with this construct failure and subsequent patient management.

Intervertebral foramen size and volume changes in low grade, low dysplasia isthmic spondylolisthesis.

STUDY DESIGN: Anatomic study. OBJECTIVE: The purpose of this study is to determine the effect of the slip amount, slip angle, and disc height on the geometry of the L5 foramen in low-grade, low-dysplasia isthmic L5 spondylolisthesis using a human cadaveric model. SUMMARY OF BACKGROUND DATA: Radicular pain and varying degrees of nerve root dysfunction are present in many adult isthmic spondylolisthesis patients and can be the result of compression of the L5 root within its foramen. METHODS: The intervertebral foramens of six L5 vertebra and sacrum cadaver specimens had a standardized pars defect created and were mounted on an adjustable frame. Plasticene molds of the foramens were made by varying sagittal translocation (0%, 12.5%, 25%, and 50%), slip angle (kyphotic 10° and lordotic 0°, 10°, 20°, and 30°), and disc height (0, 5, and 10 mm). The volume of each mold was measured. The surface area was determined by sagittally slicing on a microtome the mold and each section's face was measured by computer image analysis. RESULTS: The area and volume of the foramen at a sagittal slip of 0%, slip angle of 0°, and 0 mm disc height were used as a control. As disc height decreased from 10 to 5 mm, 10 to 0 mm, and 5 to 0 mm, the foramen area and volume significantly decreased (P < 0.05). Incremental slip percentage from 0% to 12.5%, 25%, and 50% produced significantly reduced foramen area and volumes (P < 0.05). As slip angle increased from 10° kyphosis to 0°, 10°, 20°, and 30° lordosis, foramen area and volume decreased (P < 0.05). CONCLUSION: Disc height and slip percentage had the largest effect on intervertebral foramen area and volume in this model of low-grade, low-dysplasia isthmic spondylolisthesis. Slip angle changes affected foramen area to a lesser degree. Surgical treatment strategies should consider restoration of disc height in cases with foraminal stenosis.

A novel method to determine suture anchor loading after rotator cuff repair--a study of two double-row techniques.

BACKGROUND: The addition of a lateral suture anchor fixation row to rotator cuff repairs has been shown to improve initial cuff reattachment strength and footprint area. This study evaluated the mechanical function of this lateral row by measuring suture tensions at the individual anchor sites. MATERIALS AND METHODS: Eight cadaveric shoulders underwent simulated rotator cuff repairs, using either double row or suture-bridge repair techniques. Suture tensions at each anchor were measured for several static, simulated shoulder positions relevant to postoperative patient management by specially designed instrumented anchors. RESULTS: Significantly greater suture tensions were measured at the medial anchor sites than at the lateral sites for the double-row (p < 0.001), as well as the suture-bridge constructs (p < 0.016). In the double-row technique, the lateral row sustained 21% (range, 6 to 31) of the total anchor load; whereas, in the suture-bridge technique, the lateral row sustained 33% (range, 8 to 42). Shoulder abduction from 45 degrees to 60 degrees had little effect on anchor tensions; 20 degrees internal and external rotation significantly (p = 0.032) increased loads on the anterior and posterior anchors. CONCLUSIONS: Forces are transmitted through the entire body of the tendon at its humeral fixation, loading the lateral anchors, as well as the medial row, for the two fixation techniques studied. These findings explain the higher laboratory-obtained fixation strengths of double-row techniques. The magnitude and distribution of anchor suture tensions could have important implications for postoperative positioning and activity.

Effects of lag screw design and lubrication on sliding in trochanteric nails.

This study compared the sliding characteristics of three lag screw designs used with trochanteric nails and determined the effects of lubrication on sliding. They were tested by an established method to measure initiation and ease of lag screw sliding. These tests were then repeated with calf serum lubrication. There were significant differences (p < 0.05) between the loads required to initiate lag screw sliding that appeared to be related to design. Screw sliding was similar for all three designs; however, the presence of lag screw locking slots affected sliding in that region. Lubrication did not affect either parameter. Lag screw design aspects, such as diameter and, particularly, surface finish, affect sliding. Due to the small contact area between the lag screw and nail creating high interface stresses, lubrication had no effect on lag screw sliding.

Suture anchor loading after rotator cuff repair: effects of an additional lateral row.

HYPOTHESIS: Our initial hypothesis was that the medial row of double-row rotator cuff repair techniques would bear most of the load on the repaired cuff. MATERIALS AND METHODS: Six cadaver shoulders underwent simulated rotator cuff repairs using sequential single row, double-row, and suture-bridge repair techniques. Suture tensions at each anchor were measured for several static, simulated shoulder positions by specially designed, instrumented anchors. RESULTS: Significantly greater suture tensions were measured in the anchors in a single row repair construct than either the double row repair or suture bridge repair construct (P < .001). In the double-row and suture bridge techniques, there was no apparent difference in the loads born by the medial and lateral row anchors. Shoulder abduction from 45 degrees to 60 degrees had little effect on anchor tensions; 45 degrees internal and external rotation significantly (P = .032) increased loads on the anterior and posterior anchors by at least 125%. DISCUSSION: Forces are transmitted through the entire portion of the tendon at its humeral fixation, loading the lateral anchors as well as the medial row for the techniques studied. This "load sharing" can explain the higher fixation strengths of double row techniques seen experimentally. CONCLUSION: The magnitude and distribution of anchor suture tensions could have important implications for lateral row fixation devices and post-operative positioning and activity. LEVEL OF EVIDENCE: Basic Science.

Femoral component positioning in resurfacing arthroplasty--effects on cortical strains.

Previous studies have suggested that femoral component positioning in resurfacing arthroplasty may affect strains in the femoral neck that could lead to decreased implant longevity. A strain gaged, Sawbones model was used to determine the femoral neck strains for a variety of resurfacing head translations and angulations. We found that head positions affected strain distributions, most positions leading to increased neck strains, often over 100%, with the exception being a varus head position where the superior neck strains decreased over 50%. Although the clinical meaning of these findings is unclear, it could be of concern for stress-shielding or fatigue fracture of the femoral neck.

Mechanical evaluation of unipolar hip spacer constructs.

The strengths of 3 hip spacer constructs--Steinmann pins, a short intramedullary nail (both cement-incorporated), and a Charnley prosthesis--were determined and compared with the strength of a commercially available hip spacer. The hip prosthesis construct was more than twice as strong as the other 2 constructs and was equivalent in strength to the commercial spacer. For spacer applications in which limited weight-bearing is anticipated, the hip prosthesis construct appears more efficacious, but its pros and cons should be compared with those of the commercial product.

Conformational instability of the taylor spatial frame: a case report and biomechanical study.

BACKGROUND: The Taylor spatial frame (TSF) is a second-generation circular fixator used for limb lengthening and deformity correction. While treating a patient for pseudoarthosis, gross instability of a particular TSF construct was observed. A subsequent mechanical study of the TSF was then performed to better understand how its configurations affect frame stability. METHODS: Various conformations of the TSF were made and tested in compression, bending, and torsional loading. RESULTS: Frame stability was significantly compromised in compression and bending when shorter struts were used and the ring-strut angles were less than 30 degrees. Torsional stability was not significantly affected. Some minor instabilities were noted with an angular ring offset displacement of 25 degrees. CONCLUSIONS: Ring-strut angle, a critical factor in truss mechanics, seems to have the greatest influence on stability for the TSF. We recommend that adequate length struts be chosen when constructing a TSF so that ring-strut angles of 30 degrees or less are avoided. Clinical relevance pertains to use of the TSF in atypical conformations, especially in the pediatric population requiring deformity correction.

Integrity of the stem-cement interface in THA: effects of stem surface finish and cement porosity.

No consensus exists for the optimal surface finish on cemented total hip prosthesis stems. The purpose of this study was to determine the effects of stem finish and interfacial cement porosity on the integrity of the stem-cement interface. Simulated stems made of Co-Cr, having polished or matte surfaces, at room temperature or heated to 37 degrees C, were cemented into Sawbones simulated femurs. Push out testing of the stem-cement interface was performed immediately after cement polymerization and after two aging periods in 37 degrees C saline or 37 degrees C air, and the extent of interfacial porosity at the stem-cement interface was determined. Polished stems exhibited an average 60% greater interfacial strength than that of matte stems initially and up to 240% after aging treatments. Cement porosity at the stem-cement interface and incomplete cement interdigitation into surface asperities on matte stems likely allowed saline penetration into the stem-cement interface during wet aging, resulting in a rapid decrease of shear strength. Stem preheating to 37 degrees C virtually eliminated interfacial pores and resulted in greater shear strengths regardless of surface finish. Polished stem surfaces with stem preheating provided the best interfacial shear strength and sealing ability against saline penetration into the stem-cement interface and could result in increased longevity of stem fixation.

Evaluation of a novel, nonspanning external fixator for treatment of unstable extra-articular fractures of the distal radius: biomechanical comparison with a volar locking plate.

PURPOSE: To compare the stability of a novel, nonspanning external fixator with a standard volar locked plate for treatment of unstable distal radius fractures. METHODS: A simulated, unstable, extra- articular distal radius fracture was created in six matched pairs of fresh frozen human distal radii. One of each pair was treated with a nonspanning external fixator [Mirza Cross Pin Fixator (CPX), A.M. Surgical Inc. Smithtown, NY] and the other was treated with a volar locked plate [Distal Volar Radial Plate (DVR), Hand Innovations, Miami, FL]. Each specimen was axially loaded in central, dorsal, and volar locations, loaded in cantilever bending in volar to dorsal, dorsal to volar, and radial to ulnar directions and loaded in torsion. Load-displacement curves were generated to determine the construct stiffness for each loading schema, with comparisons made between the two treatment groups. Specimens were then cyclically loaded with 50 N axial loads applied for 1,000 and 10,000 cycles. Measurement of construct stiffness was repeated and comparisons made both between the two treatments and within treatments to their precycling stiffness. RESULTS: There was no significant difference in the mechanical stiffness of the nonspanning external fixator and the volar locking plate after axial loading in any of the loading modalities. Cyclic loads of 1,000 and 10,000 cycles resulted in no significant difference in construct stiffness between the nonspanning external fixator and volar locked plate. However, the nonspanning external fixator demonstrated decreasing stiffness after cyclic loading with 10,000 cycles (p < 0.02). CONCLUSION: This study demonstrated no significant difference in the mechanical stiffness of the CPX nonspanning external fixator and volar locked plate in a cadaveric fracture model. Both constructs appear to be biomechanically equivalent in this experimental model; however, this is only one factor in the choice of fixation device for the management of unstable distal radius fractures.