There is an ever increasing drive to increase the range of movement after total knee arthroplasty (TKA). Some cultures all but demand it, some surgeons feel their technique enhances it, and many companies state that their design promotes it. The major goal of a primary TKA is to relieve pain whilst providing an adequate range of movement for activities of daily living. Range of movement following TKA, however, determines whether patients can manage high flexion activities such as crouching, kneeling or rising from the floor.
There are many factors other than prosthetic design, which influence flexion after TKA 1. Female gender, higher body mass index, previous surgery and other co-morbidities are associated with reduced flexion 2 while intra-operative factors such as component malposition, 'overstuffing' the patellofemoral joint by inserting oversized components, inadequate flexion gap balancing, failure to remove posterior osteophytes and inattention to patellofemoral tracking and thickness have been reported to have a negative effect 1. Most consistently however, the literature reports pre-operative range of movement as the best predictor of post operative range of movement 3. It has been suggested that those patients whose preoperative range of flexion was less than the mean tend to gain flexion postoperatively, whereas those with preoperative flexion greater than the mean tended to lose flexion 4. Thus there is a migration towards a mean post operatively from a broad spectrum of flexion before surgery.
The debate between whether to preserve or substitute the posterior cruciate ligament in total knee arthroplasty in order to gain flexion continues with several reviews of the literature finding insufficient evidence to recommend either 5. An implant which can achieve a deep flexion angle whilst maintaining stability has been the focus of research lately. These high-flexion prostheses include features such as reduced posterior femoral condylar radii, modifications in tibial and femoral components to accommodate extensor mechanisms with deep flexion and facilitation of physiological posterior femoral rollback. This article aims to summarise the current published studies comparing high flexion designed TKAs versus standard TKAs in order to determine whether we should be using these implants for high flexion demanding patients.
What is a high-flex knee?
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| Figure 1: Design features of the NexGen LPS-Flex knee. Detailing the extra 2mm bone resection of the posterior condyles. Taken from Huang et al. The early results of high flex TKA. J Arthroplasty 2005. |
The initial concept came from Walker and Sathasivam 7 who reported that by increasing the thickness of the posterior condyle of the femoral component may overcome the edge loading and resultant increased posterior polyethylene wear and damage. The high flex design has a smaller femoral radius of curvature and thicker posterior condyle. In theory the smaller femoral radii of curvature increases the contact area between the posterior femoral condyle and the tibial insert. In addition to the thicker posterior condyle the NexGen LPS Flex (Zimmer, Warsaw, IN, USA) has a modified cam/post mechanism and an anterior cut out slope in the polyethylene insert to allow increased jump distance whilst avoiding dislocation at deep flexion angles.
Meta analysis
A recent review of the literature revealed several studies (Table 1) evaluating the effectiveness of high flexion implants in TKA 7-14. However of these nine studies only three 12, 14, 15 were prospective randomised controlled trials whilst the others were observational studies 7-11, 13. Five studies 7, 9, 10-12 reported greater flexion or range of motion with the High Flexion implant; however, the methodological rigour was questionable with inadequate blinding, flawed participant selection, short follow-up periods and functional outcomes which lacked sensitivity.