Bradley R. Merk

Two-part surgical neck fractures of the proximal part of the humerus. A biomechanical evaluation of two fixation techniques.

BACKGROUND Successful internal fixation of fractures of the surgical neck of the humerus can be difficult to achieve because of osteopenia of the proximal aspect of the humerus. The purpose of this study was to compare the biomechanical stability of a proximal humeral intramedullary nail and a locking plate for the treatment of a comminuted two-part fracture of the surgical neck in a human cadaver model. METHODS Twenty-four cadaveric humeri were instrumented with use of either a titanium proximal humeral nail (PHN) or a 3.5-mm locking compression plate for the proximal part of the humerus (LCP-PH). The specimens were matched by bone mineral density and were separated into four experimental groups with six humeri in each: PHN bending, LCP-PH bending, PHN torsion, or LCP-PH torsion. Comminuted fractures of the surgical neck were simulated by excising a 10-mm wedge of bone. Bending specimens were cyclically loaded from 0 to 7.5 Nm of varus bending moment at the fracture site. Torsion specimens were cyclically loaded to +/-2 Nm of axial torque. The mean and maximum displacement in bending, mean and maximum angular rotation in torsion, and stiffness of the bone-implant constructs were compared. RESULTS In bending, the LCP-PH group demonstrated significantly less mean displacement of the distal fragment than did the PHN group over 5000 cycles (p = 0.002). In torsion, the LCP-PH group demonstrated significantly less mean angular rotation than did the PHN group over 5000 cycles (p = 0.04). A significant number of specimens in the PHN group failed prior to reaching 5000 cycles (p = 0.04). The LCP-PH implant created a significantly stiffer bone-implant construct than did the PHN implant (p = 0.007). CONCLUSIONS The LCP proximal humeral plate demonstrated superior biomechanical characteristics compared with the proximal humeral nail when tested cyclically in both cantilevered varus bending and torsion. The rate of early failure of the proximal humeral nail could reflect the high moment transmitted to the locking proximal screw-bone interface in this implant. CLINICAL RELEVANCE The high failure rate in torsion of the proximal humeral nail-bone construct is concerning, and, with relatively osteoporotic bone and early motion, the results could be poor.

Vertically oriented femoral neck fractures: Mechanical analysis of four fixation techniques

Objective: Femoral neck fractures in young individuals are typically high angled shear fractures. These injuries are difficult to stabilize due to a strong varus displacement force across the hip with weight bearing. The purpose of this study was to compare the biomechanical stability of four differing fixation techniques for stabilizing vertical shear femoral neck fractures. Methods: Vertical femoral neck fracture stability was assessed using 4 surgical constructs in 32 cadaveric femurs: 7.3 mm cannulated screws placed in a triangular configuration (group 1), a 135-degree dynamic hip screw (group 2), a 95-degree dynamic condylar screw (group 3), and a locking proximal femoral plate (group 4). The 4 groups were matched for mean bone density and each specimen was tested under incremental loading, cyclical loading, and loading to failure. The modes of fixation failure were recorded for each specimen and the mean group stiffness, failure loads, and failure energies were calculated. Results: All 8 specimens failed during incremental loading in group 1. Five of 8 constructs failed with incremental loading, and 3 failed with cyclical testing in group 2. The combined 16 specimens in groups 3 and 4 survived both incremental and cyclical loading. The differences in stiffness, failure loads, and failure energies between the 4 groups were statistically significant (P < 0.001). The strongest construct was the locking plate and the weakest construct was the 7.3-mm cannulated screw configuration. The cannulated screw configuration group failed as the screws backed out of the femoral head and by varus collapse of the osteotomy; the fixed angled devices all failed at the bone-implant interface. Conclusions: The strongest construct for stabilizing a vertical shear femoral neck fracture is the proximal femoral locking plate, followed in descending order by the dynamic condylar screw, the dynamic hip screw, and the 3 cannulated screw configuration.

Comparison of lateral locking plate and antiglide plate for fixation of distal fibular fractures in osteoporotic bone: a biomechanical study.

Objectives: The purpose of this study was to compare the biomechanical properties of posterolateral antiglide plating and lateral locked plating for fixation of displaced short oblique fractures of the fibula in osteoporotic bone. Methods: Short oblique fractures of the distal fibula at the level of the syndesmosis were simulated with a fibular osteotomy and ligamentous sectioning in 18 paired fresh frozen ankles. The fractures were fixed with either a lateral locking plate with an independent lag screw or a posterolateral antiglide plate with a lag screw through the plate. The specimens were tested under a torsional load to failure. The torque to failure, angular rotation at failure, and construct stiffness of the two groups were compared. Results: The torque to failure and construct stiffness were significantly greater on the side with the posterolateral antiglide plate than on the side with the the lateral locking plate (P = 0.01 and 0.005, respectively). Conclusions: The posterolateral antiglide plate demonstrated improved biomechanical stability as compared to the lateral locking plate in osteoporotic bone. In situations where fixation needs to be optimized, use of an antiglide plate may be favored over a lateral locking plate construct.

Gel scaffolds of BMP-2-binding peptide amphiphile nanofibers for spinal arthrodesis.

Peptide amphiphile (PA) nanofibers formed by self-assembly can be customized for specific applications in regenerative medicine through the use of molecules that display bioactive signals on their surfaces. Here, the use of PA nanofibers with binding affinity for the bone promoting growth factor BMP-2 to create a gel scaffold for osteogenesis is reported. With the objective of reducing the amount of BMP-2 used clinically for successful arthrodesis in the spine, amounts of growth factor incorporated in the scaffolds that are 10 to 100 times lower than that those used clinically in collagen scaffolds are used. The efficacy of the bioactive PA system to promote BMP-2-induced osteogenesis in vivo is investigated in a rat posterolateral lumbar intertransverse spinal fusion model. PA nanofiber gels displaying BMP-2-binding segments exhibit superior spinal fusion rates relative to controls, effectively decreasing the required therapeutic dose of BMP-2 by 10-fold. Interestingly, a 42% fusion rate is observed for gels containing the bioactive nanofibers without the use of exogenous BMP-2, suggesting the ability of the nanofiber to recruit endogenous growth factor. Results obtained here demonstrate that bioactive biomaterials with capacity to bind specific growth factors by design are great targets for regenerative medicine.

Mechanical evaluation of fracture fixation augmented with tricalcium phosphate bone cement in a porous osteoporotic cancellous bone model

Objective: The purpose of this study was to examine the effects of resorbable bone cement on screw and plate-screw fracture fixation in a porous osteoporotic bone model. Methods: Experiment 1: Screw pullout strength was assessed for 4 sets of 4.5-mm cortical screws inserted into a synthetic osteoporotic cancellous bone model, including screws inserted without cement augmentation (control), screws augmented with tricalcium phosphate (TCP) bone cement (Norian SRS; Synthes USA, Paoli, PA), and screws augmented with polymethylmethacrylate. Experiment 2: The effects of cement augmentation on plate-screw fixation strength were examined by performing cantilever bending tests on 4 sets of 8 plate-screw constructions, including nonaugmented and TCP-augmented standard and locked screw-plate constructions in a similar bone model. Results: Experiment 1: Cement augmentation with both TCP and polymethylmethacrylate increased screw pullout strength from a porous osteoporotic cancellous bone model by about 4-fold (P < 0.05), and there was no significant difference between the 2 cements (P > 0.1). Experiment 2: Fixation strength was 1.5 times higher for locked plates compared with standard plates when neither was augmented with cement (P = 0.07). Cement augmentation with TCP improved fixation strength by 3.6 times for a standard plate-screw construction (P < 0.05) and 3.3 times for a locked plate-screw construction (P < 0.05). The most stable construction was the TCP-augmented locked plate, in which a 5-fold increase was observed compared with that of standard plates without TCP (P < 0.05). Conclusions: This study indicates augmenting screws with TCP cement during osteosynthesis improves fixation strength in an osteoporotic cancellous bone model. Clinical Relevance: In fracture situations in which osteoporotic bone makes screw and screw-plate fixation tenuous, screw augmentation with TCP cement should be considered as adjunct treatment.

Clinical and radiographic comparisons of two different radial head implant designs

BACKGROUND There is little comparative data to guide implant choice for radial head replacements. The purpose of this study was to evaluate the clinical and radiographic results between patients who received a smooth-stemmed bipolar radial head implant and patients who received an in-growth monopolar prosthesis. METHODS Twenty-seven patients requiring a metallic radial head implant in the management of acute or chronic elbow trauma were evaluated. Fourteen patients received a smooth-stemmed bipolar prosthesis and 13 patients received a press-fit monopolar prosthesis. Patients returned for follow-up at an average of 33 months (range, 18-57). Outcome assessments included joint motion, elbow stability, grip strength, pain, the Mayo Elbow Performance Index, and the Disability of Arm, Shoulder and Hand questionnaire. Radiographs were reviewed for joint congruence, ectopic bone, periprosthetic osteolysis, degenerative arthritis, and capitellar wear, and selected patients were tested for inflammatory markers and metal ion levels. RESULTS The differences between patient groups for elbow flexion and forearm pronation averaged 10° or less. There were no other pertinent differences between groups for standardized patient and examiner-determined outcomes. There was a trend for ectopic bone to develop more commonly around the smooth-stemmed implants, while periprosthetic osteolysis was more pronounced in cases with the press-fit design. Inflammatory markers were normal, and metal ion levels did not exceed values reported for a well-functioning hip arthroplasty. CONCLUSION Outcomes at short- to mid-term follow-up were similar with either implant design. Loosening of a press-fit prosthesis may lead to extensive osteolysis, but of undetermined clinical consequence.

Extra-articular distal tibia fractures: a mechanical evaluation of 4 different treatment methods.

Objective: This cadaveric biomechanical study compared the mechanical properties of standard plating (SP), locked plating (LP), intramedullary nailing (IMN), and angular stable intramedullary nailing (ASN) for the treatment of axially unstable distal tibia metaphyseal fractures (OTA type 43.A3) with an intact fibula. Methods: A distal tibia metaphyseal fracture was created in 30 fresh frozen cadaveric specimens by performing an osteotomy 30 mm above the plafond. The fibula was left intact. Specimens were divided into 4 groups. Specimens underwent fracture fixation with a standard distal tibia plate, a medial locked plate, an intramedullary nail, or an angular stable intramedullary nail. Specimens were loaded vertically along the tibial axis to 700 N, followed by cyclical loading at 700 N for 10,000 cycles, and then to failure. Results: The IMN group demonstrated greater stiffness and load to failure than the LP group, which was greater than the SP group. The ASN group was not different in terms of stiffness and load to failure from the LP group for the number of specimens tested. The IM group required the greatest energy to failure, and all groups were significantly greater than the SP group. Conclusions: Under axial loading conditions with an intact fibula, both IMN and LP provide stable fixation. There was no advantage to the use of an ASN over a standard IMN. IMN resulted in the highest stiffness, load to failure, and failure energy for OTA type 43.A3 fractures with as little as 3 cm of distal bone stock.

A fatigue life analysis of small fragment screws.

Objectives To conduct a comparative fatigue analysis of several commonly used small fragment screws. Design Biomechanical laboratory study. Setting Research laboratory. Main Outcome Measurements A fatigue life analysis of seven different types of small fragment screws was conducted using a Wohler fatigue-testing machine. Four different types of 3.5-millimeter cortical screws were subjected to fatigue analysis. These included solid stainless steel screws from Synthes Ltd. (core diameter 2.4 millimeters), Zimmer Inc. (core diameter 2.4 millimeter), and Smith and Nephew Richards Inc. (core diameter 2.4 millimeters) and cannulated stainless steel screws from Synthes Ltd. (core diameter 2.5 millimeters). In addition, three types of 4.0-millimeter cancellous screws were tested. These included stainless steel screws from Synthes Ltd. (core diameter 1.9 millimeters), titanium screws from Synthes Ltd. (core diameter 2.0 millimeters), and titanium alloy screws from DePuy-Ace (core diameter 2.8 millimeters). Fatigue lives, as reflected by mean cycles to failure, were compared. Results The four types of cortical screws had longer fatigue lives than the Synthes cancellous screws did (p < 0.001) but shorter fatigue lives than the DePuy-Ace cancellous screws did (p < 0.0001). Among the cortical screws, the cannulated and solid Synthes screws and the solid Zimmer screws did not differ statistically. The Smith and Nephew Richards cortical screws failed at statistically fewer cycles than the Synthes solid and cannulated cortical screws did (p < 0.003) but did not statistically differ from the Zimmer screws. The DePuy-Ace titanium alloy cancellous screw had the longest fatigue life of the tested implants by a large margin (p < 0.0001). The Synthes pure titanium and stainless steel cancellous screws did not significantly differ. Conclusions This analysis supports core diameter as the principal factor determining fatigue life as the results paralleled implant geometry. This design modification to improve bending and fatigue strength may come at a price to pullout strength, however, because of a decreased major-to-minor diameter and increased pitch. Cortical screws differed in fatigue performance despite identical dimensions, presumably highlighting the importance of implant processing and machining. Cannulated cortical screws performed well relative to solid screws, thereby supporting their clinical use. Pure titanium and stainless steel cancellous screws performed similarly in fatigue despite differing material properties, presumably because of geometric design differences. This report highlights some of the differences in the in vitro fatigue performance among several commonly used small fragment screws.

Extracapsular Placement of Distal Tibial Transfixation Wires

BACKGROUND Treatment of tibial plafond fractures with external fixation may involve use of transfixation wires within the periarticular region. Pin track infections that develop along wires placed intracapsularly may lead to joint infection. To our knowledge, there have been no previous investigations assessing the circumferential reflection of the ankle capsule or the potential for communication between the distal tibiofibular joint and the tibiotalar joint. The purpose of this study was to define these anatomic entities to provide guidelines for safe extracapsular placement of distal tibial wires. METHODS Twelve fresh-frozen cadaveric ankles and three ankles of living human volunteers were utilized for this study. High-resolution magnetic resonance imaging was performed on each ankle after pressurized distention of the joint capsule with gadolinium solution. The perpendicular distance from the subchondral bone at the joint line to the capsular synovial reflection was measured with use of a verified technique. The cadaveric ankles were sectioned, the capsular synovial reflections were measured by investigators who were blinded to the imaging results, and the corresponding measurements were compared. RESULTS The anterolateral capsular synovial region displayed the most proximal reflection in all specimens (mean, 9.3 mm; maximum, 12.2 mm). The anteromedial region displayed less reflection (mean, 3.3 mm; maximum, 5.5 mm). All posteromedial and posterolateral synovial reflections were <or=2 mm. Capsular synovial reflections proximal to the medial and lateral malleoli were negligible. In all ankles, the distal tibiofibular joint communicated with the tibiotalar joint and had a maximum proximal extension of 20.6 mm. CONCLUSIONS Placement of distal tibial transfixation wires >12.2 mm from the subchondral surface of the plafond avoids penetration of the capsule. The distal tibiofibular joint communicates with the tibiotalar joint and thus should not be penetrated, to ensure extracapsular placement of the wires.

Intra-articular distal femur fracture extending from an expanded femoral tunnel in an anterior cruciate ligament (ACL) reconstructed knee: a case report.

Anterior cruciate ligament (ACL) reconstruction is a popular surgery with overall good results. However, postoperative fracture is a serious, albeit, rare complication. Femoral tunnel lysis may cause a stress riser effect leading to distal femur fractures after ACL reconstruction. We report a case in which a patient who underwent ACL reconstruction surgery 18 months before a low-energy twisting mechanism suffered a distal femur fracture requiring open reduction internal fixation.