Although rare, periprosthetic fractures remain a significant complication after total knee arthroplasty (TKA), occurring in 0.3% to 2.5% of cases.1-4 Hirsh and colleagues5 were among the first to suggest that anterior femoral notching during TKA was a potential risk factor for postoperative periprosthetic femoral fracture because notching may weaken the anterior femoral cortex. Anterior femoral notching, a cortex violation occurring during an anterior bone cut, occurs in up to 30% of cases.6 Using a theoretical biomechanical model, Culp and colleagues1 found that increasing the depth of the notch defect into the cortex led to reduced torsional strength. In more recent, cadaveric biomechanical studies, notching of the anterior femoral cortex decreased torsional strength by up to 39%.7,8 Contrary to these biomechanical studies, a retrospective study evaluating 1089 TKAs using 2 implant designs (Anatomic Graduated Component, Biomet and Legacy, Zimmer) demonstrated no significant effect of anterior femoral notching with respect to incidence of supracondylar femur fractures.6 That study, however, did not address whether implant design is associated with a differential risk for fracture in the presence of anterior notching.
Previous biomechanical studies have primarily investigated cruciate-retaining (CR) femoral components and properties with respect to anterior notching, even though the posterior-stabilized (PS) design is used more often in the United States.1,7 According to a Mayo Clinic survey, TKAs with a PS design increased from <10% in 1990 to almost 75% by 1997.9 Today, there is little to no consensus about which implant is better, and use of one or the other depends largely on the surgeon and varies widely between countries and regions.10 PS designs require more bone resection and demonstrate prosthesis-controlled rollback during flexion, whereas CR designs preserve more bone and achieve posterior stabilization via the posterior cruciate ligament.11 Despite these differences in design and mechanics, a 2013 Cochrane review of TKA design found no clinically significant differences between CR and PS with respect to pain, range of motion, or clinical and radiologic outcomes.10 The reviewers did not specifically address periprosthetic fractures associated with either femoral notching or TKA design, as they could not quantitatively analyze postoperative complications because of the diversity of reports. Given the limited number of reported cases, a review of radiographic findings pertaining to the characteristics of supracondylar fractures in anterior femoral notching was unsuccessful.12 As the previous biomechanical studies of anterior notching used primarily CR models or no prostheses at all, a study of biomechanical differences between CR and PS designs in the presence of anterior notching is warranted.1,7,8 Therefore, we conducted a study to assess the effect of anterior femoral notching on torsional strength and load to failure in CR and PS femoral components.
Materials and Methods
Twelve fourth-generation composite adult left femur synthetic sawbones (Sawbones; Pacific Research Laboratories) were selected for their consistent biomechanical properties, vs those of cadaveric specimens; in addition, low intersample variability made them preferable to cadaveric bones given the small sample used in this study.13,14 All bones were from the same lot. All were visually inspected for defects and found to be acceptable. In each sample, an anterior cortical defect was created by making an anterior cut with an undersized (size 4) posterior referencing guide. In addition, the distance from the proximal end of the notch to the implant fell within 15 mm, as that is the maximum distance from the implant a notch can be placed using a standard femoral cutting jig.15 Six femora were instrumented with CR implants and 6 with PS implants (DePuy Synthes). Implants were placed using standardized cuts. Before testing, each implant was inspected for proper fit and found to be securely fastened to the femur. In addition, precision calipers were used to measure notch depth and distance from notch to implant before loading. A custom polymethylmethacrylate torsion jig was used to fix each instrumented femur proximally and distally on the femoral implant (Figure 1). Care was taken to ensure the distal jig engaged only the implant, thus isolating the notch as a stress riser. Each femur was loaded in external rotation through the proximal femoral jig along the anatomical axis. Use of external rotation was based on study findings implicating external rotation of the tibia as the most likely mechanism for generating a fracture in the event of a fall.12 Furthermore, distal femur fractures are predominantly spiral as opposed to butterfly or bending—an indication that torsion is the most likely mechanism of failure.16 With no axial rotation possible within the prosthesis, increased torsional stress is undoubtedly generated within adjacent bone. Each specimen underwent torsional stiffness testing and then load to failure. Torsional stiffness was measured by slowly loading each femur in external rotation, from 1 to 18 Nm for 3 cycles at a displacement rate of 0.5° per second. Each specimen then underwent torsional load-to-failure testing on an Instron 5800R machine at a rate of 0.5° per second. Failure was defined as the moment of fracture and subsequent decrease in torsional load—determined graphically by the peak torsional load followed immediately by a sharp decrease in load. Stiffness was determined as the slope of torque to the displacement curve for each cycle, and torque to failure was the highest recorded torque before fracture. Fracture pattern was noted after failure. A sample size of 6 specimens per group provided 80% power to detect a between-group difference of 1 Nm per degree in stiffness, using an estimated SD of 0.7 Nm per degree. In our statistical analysis, continuous variables are reported as means and SDs. Data from our torsional stiffness and load-to-failure testing were analyzed with unpaired 2-sample t tests, and P < .05 was considered statistically significant.
