Yifei Dai, PhD
Weimin Yue, PhD
Laurent Angibaud, Dipl. Ing.
In the modern era of total knee arthroplasty (TKA), comprehensive consideration of an individual’s bone and soft tissue characteristics has gained more focus. It is important to select a prosthesis that best fits the native morphology of the targeted patient population. Studies have shown that mediolateral (ML) oversizing of the components can compromise the clinical outcomes of the surgery, such as lower functional scores, less range of flexion, and may account for 27% of postoperative knee pain, possibly due to irritation of the soft-tissue around the knee.1-3
In the femur, the goal of minimizing component overhang can be complicated by other surgical considerations with unintended consequences. For example, ML overhang can be avoided by undersizing or using a narrower femoral component. In the undersizing circumstance, the undersized anteroposterior (AP) dimension of the component can increase flexion laxity or cause anterior notching. Although these situations may be resolved by additional femoral resections to prepare for the undersized femur, the resulting joint line will inevitably be elevated, which can negatively impact patellofemoral kinematics, increase the incidence of instability, or decrease knee flexion.4-8 Studies have identified that an overhang of more than 3-4mm is clinically important. In a 2010 study, Mahoney et al. reported that more than 3mm of component overhang can increase the risk of clinically important knee pain by 90%.2 A recent investigation by Chung et al. concluded that more than 4mm of overhang can significantly lower the maximum flexion angle postoperatively.9
Many studies have evaluated the ML morphological fit of modern femoral implants. Most analyses were focused on the component fit at limited locations, typically at the distal resection area.2,9,10,11,12 However, complex variations in femoral morphology have been reported, which are not limited to the distal portion of the femur.13 The design and development of the Exactech’s Truliant® Knee System employed a series of comprehensive morphological studies to minimize ML overhang of the femoral component. This present study computationally assessed the component of the Truliant femoral components superimposed on a dataset of medium to small knees.
MATERIALS AND METHODS
Digital femoral surface models of 30 Chinese (11M/19F) and 24 Caucasian (5M/19F) knees were selected from a CT scan based virtual bone database. The selection included all the right femora in the database that had an AP dimension of no more than 57.9mm measured from the anterior cortex point to the tangent plane of both posterior condyles (corresponding to Truliant component sizes of 3 and under).
Component Sizing and Placement
Each femur was virtually resected in accordance with its proper component size following the Truliant anterior referencing surgical technique (Unigraphics NX, Siemens PLM Software, Plano, TX, USA). Virtual implantation of the femoral component was performed by a computational algorithm. The algorithm first lateralized the femoral component such that it aligned with the bony resection at the AP mid-point of the distal cut (Location 1, Figure 1). The fit of the femoral component was then measured at 10 anatomical locations on the distal, anterior chamfer, and anterior resection areas (Matlab, Mathworks Inc, Natick, MA, USA) (Figure 1). In some instances, if overhang of more than 3mm (clinically important overhang) was detected at any location, a secondary algorithm slightly adjusted the ML position of the component by allowing an ML translation in the opposite direction (no more than 3mm), optimizing the fit by minimizing both the number of locations and the severity of overhang.
The subsequent ML fit of the component was assessed at the 10 anatomical locations. The incidence of the component extending beyond the resection bony profile were identified. The associated amount of mismatch between the component and bony resection was recorded. Clinically important overhang was defined when the component extended more than 3mm beyond the bony profile.
Figure 2 illustrates the two dimensional unfolded or “flattened” profiles of the Truliant femoral component sizes used in this study (from location 1 to the proximal tip of the flange), overlaid with the associated flattened bony resection profiles according to the component placement. Across genders and ethnicities, the Truliant femoral component design consistently minimized clinically important overhang (Table 1, Figure 2, 3). Only one knee (Chinese, female) had a negligible additional amount (0.1mm) of overhang over 3mm at location 2 lateral.
Exactech’s Truliant Knee System continues the successful evolution of the Optetrak Logic® Knee System, with renewed focus on the restoration of patient’s natural anatomy in addition to the clinically proven, patented legacy design for articular and patellar performance.14-16 The findings of this study showed excellent fit among medium to small-sized knees studied in the dataset. Only one knee that had an overhang in excess of the clinically important threshold. Contrary to the common belief that the fit of western originated knee designs may be compromised in other populations, especially Asian patients,9,10,17 the Truliant femoral design was shown to provide equally good fit for both ethnic groups studied. In addition, although it has been reported that component fit in female knees is inferior to that in male knees,2,10 the Truliant femoral design did no demonstrate gender-based differences in terms of clinically important overhang incidence.
Computational assessment of the femoral fit of Exactech’s Truliant femoral component design demonstrated minimal incidence of clinically important overhang in both gender and ethnic groups investigated. The data confirmed that the sizing of the Truliant femoral component respects the anatomy of the distal femur, potentially minimizing the risk of TKA complications related to femoral component overhang.
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