Total knee arthroplasty (TKA) is a highly successful operation with high patient satisfaction and good quality of life ^[1]. The classic posterior stabilized (PS) design makes use of an intercondylar “box” cut on its femoral component for effective kinematics. This box cut houses the post and cam mechanism which prevents anterior sliding of the femur on the tibia in a PS knee where the posterior cruciate ligament (PCL) is resected. Designs of various manufacturers vary in terms of the box but have tended toward constant box size throughout a range of knee sizes [2,3]. This is a concern for smaller femur sizes where even a small prosthesis has a box cut equivalent to its larger counterpart. This has led to more von Mises stresses experienced by the femur and ultimately leading to intraoperative fractures or iatrogenic fractures ^[2]. The incidence of intraoperative fracture is around 0.4%–2.2% ^[2]. Retrospective studies have shown that this complication tends to occur more in the older female population during bone preparation and hammering of components and with PS knee. To overcome this problem, newer generation implants have come up with a new design in their femoral component where box cut ranges according to the size of the prosthesis. The purpose of the present study was to compare the quantity of intercondylar bone removed during femoral box osteotomy for implantation of the femoral component of three contemporary newer generations PS TKA designs Attune PS (DePuy), Anthem (Smith and Nephews), and NexGen Legacy (Zimmer) with the older version from the same manufacturers.

We have compared PS total knee replacement prosthesis of the following: PFC Sigma with the newer system Attune PS of DePuySynthes, NexGen Legacy (newer system) with Persona of Zimmer, and Smith and Nephew’s newer system Anthem with Genesis in terms of the amount of bone resected during femoral box cut for the smaller sized implants. The height and width vary accordingly and so do the volume of bone resected in each of these systems.

A comparison was made between previous generation implants of the same company with newer generation implants in terms of their box volumes for smaller sizes. We have taken into account the smallest size of each company. DePuy Sigma PFC size 2 has been compared with the 1.5 size of Depuy Attune. Similarly, Zimmer NexGen sizes B and C compared with Persona sizes 1 and 2 and Smith and Nephew Anthem sizes 1N and 2N compared with Genesis 1N and 2N. Direct measurements of removed bone were acquired for DepuySynthes (Fig. 1a) and Smith and Nephew (Fig. 1b and c), and indirect measurements of removed bone were acquired for the rest by measuring the size of the space for the box osteotomy on each cutting jig. Measurements were acquired 3 times for each sample using a millimeter caliper in an anterior-to-posterior (length), medial-to-lateral (width), and proximal-to distal (depth) direction. Data were collected on an electronic spreadsheet. Statistical analysis was performed using the Kruskal–Wallis test for comparison between groups, with box volume as the dependent variable. The Mann–Whitney U-test was used for pairwise comparisons. Significance was set at P < 0.017 and Bonferroni’s correction was applied.

On comparing the older generation implants with newer generation implants in terms of volume of bone resected for box cut for smaller sizes, we have come to the conclusion that newer generation implants save approximately 33% of bone that is resected from the box cut. DePuy’s Attune PS saves 27.1% of bone as compared to Sigma PS resecting 6.96 cm³ of bone (Table 1). Similarly, Zimmer’s Persona saves 40.57% of bone as compared to NexGen from the intercondylar box cut resecting 6.18 cm³ (Table 2). Smith and Nephew’s Anthem and Genesis have no difference in their box volume with both resecting 7.8 cm³ of bone (Table 3). These results are tabulated in graph form (Fig. 2).

Preservation or substitution of the PCL in primary TKA is still a controversial issue [4, 5]. Gait analysis, in vivo and in vitro studies showed a reproduction of close-to-normal knee kinematics using either solution [6, 7]. A clinical comparative study between different types of TKA (CR or PS) with identical femoral geometry showed similar mid-term outcomes with regard to a range of motion, functional outcomes, and survival rate ^[8].

The purpose of this study was to compare the minimum volume of intercondylar bone removable in newly used PS TKA designs. The cam and post mechanism requires an optimum amount of bone to be resected in PS knee. However, too much resection of intercondylar bone may create a potential stress riser in the distal femur, which may predispose to intercondylar fracture, especially in osteoporotic patients with small femora. In this study, we are trying to simply identify different intercondylar bone-saving instrumentation if a PS solution is needed.

Lombardi et al. concluded that the von Misses stress considerably increases on the medial and lateral condyle after an intercondylar box cut in small-sized femur ^[8]. It is also of utmost importance that in the smaller-sized femur, the components should be placed centrally since eccentricity may give rise to stress risers in either condyle leading to intraoperative iatrogenic fractures while hammering the prosthesis.

Furthermore, the increase in the constraint level in PS knees rather than CR knees is considered, theoretically, to be an additional stress factor on the bone and the bone-implant interface ^[9]. These effects will be exaggerated in patients with smaller size femur which is the case in Asians, especially in the presence of any additional risk factors such as osteoporosis and obesity. Small femur sizes, especially ones that would require increased distal resection or change in implant positioning, may benefit from an alternative design without the need for a cam/post mechanism. In older generations of PS knee, fixed volume of intercondylar bone was resected irrespective of the size. This would create vulnerability of iatrogenic fractures in smaller-sized femur leading to higher chances of complications. This problem is rectified by companies in newer generation implants by reducing bone resected from the box in a manner that takes away fixed percentage of bone as compared to fixed volume. This has resulted in the gradual reduction of the volume of an intercondylar box from largest to smallest size. We have also done a comparison of various implant companies and rectification done in the design and suggested the optimum amount of bone to be removed from the intercondylar box cut for smoother biomechanics and to prevent iatrogenic fracture ^[10].

Irrespective of implant size, the Attune PS (DePuy) and NexGen Legacy (Zimmer) cutting jigs always resected significantly less bone than did the jigs of older generations. There was no significant difference in the bone removed during femoral box osteotomy in the newer and older generations of Smith and Nephew.

This study does establish that the newer generation PS TKA prosthesis has had suitable modifications to remove less bone at primary TKA. This may be of significance, especially in the Asian race with anthropometric smaller dimension distal femur in preserving as much bone as possible.