What Is the Possible Impact of High Variability of Distal Femoral Geometry on TKA? A CT Data Analysis of 24,042 Knees

Abstract

Background: Previous studies analyzing femoral components of TKAs have demonstrated the limited ability of these components to accommodate size variations seen in the patient population, particularly width and femoral offset.

Questions/purposes: The purpose of this study was to use a large data set of knee CT scans (1) to determine the variations in the distal and posterior femoral geometries and to determine whether there is a correlation between distal condylar offset and posterior femoral offset as a potential parameter for symmetry/asymmetry; and (2) to evaluate what proportion of knees would have a substantial mismatch between the implant's size or shape and the patient's anatomy if a femoral component of a modern standard TKA of symmetric (sTKA) or asymmetric (asTKA) designs were to be used.

Methods: A retrospective study was performed on 24,042 data sets that were generated during the design phase for a customized TKA implant. This data set was drawn from European and US-American patients. Measurements recorded for the femur included the overall AP and mediolateral (ML) widths, widths of the lateral condyle and the medial condyle, the distal condylar offset (DCO) between the lateral and medial condyles in the superoinferior direction, and the posterior femoral offset (PFO) as the difference between the medial and lateral posterior condylar offset (PCO) measured in the AP direction. A consecutively collected subset of 2367 data sets was further evaluated to determine the difference between the individual AP and ML dimensions of the femur with that of modern TKA designs using two commercially available implants from different vendors.

Results: We observed a high degree of variability in AP and ML widths as well as in DCO and PFO. Also, we found no correlation between DCO and PCO of the knees studied. Instances of a patient having a small DCO and higher PCO were commonly seen. Analysis of the DFOs revealed that overall, 62% (14,906 of 24,042) of knees exhibited DCO > 1 mm and 83% (19,955 of 24,042) of femurs exhibited a > 2-mm difference between the lateral and medial PCO. Concerning AP and ML measurements, 23% (544 of 2367) and 25% (592 of 2367) would have a mismatch between the patient's bony anatomy and the dimensions of the femoral component of ± 3 mm if they would have undergone a modern standard sTKA or asTKA design, respectively.

Conclusions: Analysis of a large number of CT scans of the knee showed that a high degree of variability exists in AP and ML widths as well as in DCO and PFO.

Clinical relevance: These findings suggest that it is possible that a greater degree of customization could result in surgeons performing fewer soft tissue releases and medial resections than now are being done to fit a fixed-geometry implant into a highly variable patient population. However, as an imaging study, it cannot support one approach to TKA over another; comparative studies that assess patient-reported outcomes and survivorship will be needed to help surgeons decide among sTKA, asTKA, and customized TKA.

Fig. 1

The use of sTKA in knees with high PFO contributes to instability, more releases, excessive bony resections to reduce the ML gap difference, or to external rotation (red arrow) of the femoral component to close the lateral flexion gap (blue arrow).

Fig. 2 A-B

The DCO was determined by measuring the superoinferior distance between the lowest point on the lateral and medial femoral condyle in extension (A). PFO was determined by measuring the superoinferior distance between the lowest point on the lateral and medial femoral condyle in flexion (B).

Fig. 3 A-B

ML and AP measurements of all available sizes of the standard TKA designs are shown. (A) Data concerning the Attune Knee system (DePuy Synthes) are presented as bold purple dots and (B) data concerning the Persona Knee System (Zimmer Biomet Inc) are presented as blue bold dots versus respective CT data, presented as gray dots. The blue-marked area represents the area to which the investigations refer. Especially patients with high ML and AP would have suffered from sizing issues ± 3 mm if they had received a sTKA or asTKA.

Fig. 4

The number of patients versus DCO is presented: the orange area represents the DCO that can be addressed with a sTKA without requiring further adjustments. The green area represents the DCO that can be addressed with a asTKA with a set varus joint line (fixed 2.5-mm tibial offset like in the Journey Knee System [Smith & Nephew Inc]) without requiring further adjustments. More than 60% of patients with a DCO > 1 mm are not addressed by a sTKA requiring further adjustments. Fifty-six percent of patients having a DCO of < 2 mm or > 3 mm are not addressed by an asTKA with a set varus joint line (fixed 2.5-mm tibial offset like in the Journey Knee System [Smith & Nephew Inc]) requiring further adjustments.

Fig. 5

The number of patients versus DCO and PFO is presented: the difference in the distribution of the DCO and PFO demonstrates that there is no direct correlation in the offsets for the patient population analyzed.

Fig. 6

The number of patients versus PFO is presented: just 20% had a PFO of < 2 mm corresponding to approximately 3° of external rotation typically used during standard TKA to close the lateral flexion gap (orange area). Eighty-three percent of femurs exhibited PFO > 2 mm.