Distal femoral varus osteotomy for unloading valgus knee malalignment: a biomechanical analysis.

PURPOSE: To investigate the biomechanical properties of the load shifting following opening-wedge distal femoral varus osteotomies (DFVOs) and determine the osteotomy correction needed to unload the lateral compartment. METHODS: Five human cadaveric knees were tested with a load of 500 N of axial compression. Medial and lateral tibiofemoral compartment contact area and pressure were assessed utilizing a modified F-scan pressure-sensitive sensor. The knees were tested in their baseline anatomic alignment, 10° valgus malalignment and following corrective DFVOs of 5°, 10° and 15°. The load shifting effect of the various DFVO correction angles was analysed using a one-way ANOVA to determine the correction angle necessary to unload the lateral compartment. RESULTS: Gradually shifting the loading vector medially with increasing DFVO angles resulted in a decrease in the mean contact area and mean contact pressures in the lateral compartment with progressive increases in the medial compartment. The largest reduction in lateral compartment pressure and contact area was seen with the 15° osteotomy with a 25 % decrease in mean contact pressure and 20 % decrease in mean maximum contact pressure and mean contact area when compared to the 10° valgus-malaligned knee. For the 10° valgus knee, a 15° correction resulted in near-normal contact pressures and areas compared with the knee in normal anatomic alignment. CONCLUSION: Progressive unloading of the lateral tibiofemoral compartment occurred with increasing DFVO correction angles. Clinically, when performing a DFVO for valgus malalignment, surgeons should consider overcorrecting the osteotomy by 5° to restore near-normal contact pressures and contact areas in the lateral compartment rather than the traditional teaching of correcting to neutral alignment.

Suture versus screw fixation of displaced tibial eminence fractures: a biomechanical comparison.

PURPOSE: Classification and treatment of tibial eminence fractures are determined by the degree of fragment displacement. A variety of surgical procedures have been proposed to stabilize displaced fractures using both open and arthroscopic techniques. Two common fixation techniques involve use of cannulated screws and sutures tied over an anterior tibial bone bridge. We are unaware of any biomechanical studies that have compared the strength of various techniques of fixation. TYPE OF STUDY: Biomechanical study in a cadaveric model. METHODS: Seven matched pairs of fresh-frozen human cadaveric knees were stripped of all soft tissue except the anterior cruciate ligament (ACL). Simulated type III tibial eminence fractures were created using an osteotome. Fragments of each matched pair were randomized to fixation with either a single 4-mm cannulated cancellous screw with a washer or an arthroscopic suture technique using 3 No. 2 Fiberwire sutures (Arthrex, Naples, FL) passed through the tibial base of the ACL and tied over bone tunnels on the anterior tibial cortex. Specimens were then loaded with a constant load rate of 20 mm/min, and load-deformation curves were generated. The ultimate strength and stiffness were computed for each curve. The failure mode for each test was observed. A paired 2-tailed t test was used to determine the statistically significant difference between the two methods. RESULTS: Specimens fixed with Fiberwire had a mean ultimate strength of 319 N with a standard deviation of 125 N. Those fixed with cannulated screws had a mean ultimate strength of 125 N with a standard deviation of 74 N. This difference was statistically significant (P = .0038). There was no significant difference between the mean stiffness of Fiberwire constructs (63 N; SD, 50 N) and the mean stiffness of the cannulated screw constructs (20 N; SD, 32 N). The failure modes of the Fiberwire constructs included 1 ACL failure, 3 failures of suture cutting through the anterior tibial cortex, and 3 of suture cutting through the tibial eminence fragment. The single mode of failure for the cannulated screw constructs was screw pullout of cancellous bone. CONCLUSIONS: The initial ultimate strength of Fiberwire fixation of tibial eminence fractures in these specimens was significantly stronger than that of cannulated screw fixation. CLINICAL RELEVANCE: It appears that Fiberwire fixation of eminence fractures provides biomechanical advantages over cannulated screw fixation and may influence the type of treatment one chooses for patients with tibial eminence fractures.

A biomechanical comparison of a dorsal 3.5-mm T-plate and a volar fixed-angle plate in a model of dorsally unstable distal radius fractures.

OBJECTIVES: To compare the biomechanical stability of internal fixation of extra-articular, dorsally unstable distal radius fractures fixed by 1 of 2 methods, either a standard dorsal nonlocked T-plate or a volar locked fixed-angle plate. DESIGN: Biomechanical cadaveric study. SETTING: Biomechanical testing laboratory. INTERVENTION: In 6 matched pairs of fresh-frozen cadaveric specimens, a simulated unstable extra-articular distal radius fracture was created. The fractures were stabilized with either a dorsal 3.5-mm stainless steel T-plate or a titanium locked volar fixed-angle plate. Specimens were axially loaded at 5 points (centrally, volarly, dorsally, radially, and ulnarly) and then cyclically loaded for 5000 cycles with an 80 N central load. Postcyclical loading, specimens were once again axially loaded at the 5 points. MAIN OUTCOME MEASURES: Initial fixation stiffness and stiffness after midaxial cyclical loading was compared at the 5 points. RESULTS: With the volar locked fixed-angle plate, fixation was significantly stiffer than with the dorsal nonlocked T-plate for ulnar and volar loading in single-cycle testing. After cyclic loading, the locked volar fixed-angle plate maintained more of its initial stiffness than the dorsal nonlocked T-plate. The dorsal 3.5-mm stainless steel T-plate's stiffness when dorsally loaded significantly decreased after cyclical loading. CONCLUSIONS: The volar locked fixed-angle plate maintained a greater percentage of its initial stiffness after cyclic loading compared to the dorsal nonlocked plate. Also, the volar locked plate was stiffer than the dorsal nonlocked plate for all loading configurations tested except when subjected to a dorsally applied eccentric load.