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?

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.

Figure 2. Lateral views of the NexGen CR-Flex (left) and NexGen LPS-Flex (right) prostheses. Extension of the radius and thickness (2 mm) of the posterior condyle in both systems increases the articular contact area at high flexion angles and thereby increases posterior femoral translation and the range of flexion. [Pictured adapted from Functional Outcome and Range of Motion of High-Flexion Posterior Cruciate-Retaining and High-Flexion Posterior Cruciate-Substituting Total Knee Prostheses A Prospective, Randomized Study. Kim et al. JBJS Am 2009]

The results were presented as either range of flexion or range of motion measured pre operatively and at the latest follow up. Only 2 studies 14-15 measured intraoperative flexion using the drop test. Three of the seven studies which investigated the LPS-Flex (Zimmer, Warsaw, IN, USA), the single studies involving the PFC Sigma RP-F (DePuy Orthopaedics, Inc., Warsaw, IN, USA) and the Genesis II High-flex PS (Smith & Nephew, Memphis, TN, USA) showed significant gains in their respective trials.

Functional outcome measures were reported in eight studies, the most common being the Knee Society Score (KSS) in five studies 7, 9, 10, 12, 14 and the Hospital for Special Surgery (HSS) Knee Score in two studies 8,11 while one study used both 15. Nutton et al. 14 also evaluated maximal functional knee flexion measured by electrogoniometry during various tasks. None of these studies showed any significant differences between the groups with these scores (p>0.05).

Table 1 - Summary of the current published literature.

Implantused Studydesign n(patients) Follow up (years)
Huang (2005) 7 LPS Case controlled
LPS-F (retrospective) 28(28) 2
Seon (2005) 8 LPS Case Controlled 50(50) 2
Gupta (2006) 9 PFC Sigma Case controlled 50(45) 1
PFC Sigma RP-F 50(45)
Laskin (2007) 10 Genesis II PS Case controlled 40(40) 2
Genesis II High flex 40(40)
Bin (2007) 11 LPS Case Controlled 97(69) 1
LPS-F 96(72)
Weeden (2007) 12 LPS RCT 25(25) 1
LPS-F 25(25)
Ng (2008) 13 LPS Case Controlled 35(35) 2
LPS-F bilateral 35(35)
Nutton (2008) 14 LPS RCT 28(28) 1
LPS-F (Double blinded) 28(28)
Kim (2009) 15 LPS RCT, bilateral 250(125) 2
LPS-F 250(125)

n – number of implants (number of patients).RCT - randomised controlled trial.

By far the largest Level 1 prospective double blind randomised controlled study by Kim et al 15 investigated whether there was an advantage to using either a posterior cruciate-retaining or posterior cruciate-substituting design. Two hundred and fifty patients received a high-flexion posterior cruciate retaining prosthesis in one knee and high-flexion posterior stabilised total knee prosthesis in the contralateral knee. Patients were assessed clinically by determining range of motion, both radiographically and functionally, with use of the knee rating systems of the Knee Society and the Hospital for Special Surgery. In addition, each patient completed the Western Ontario and McMaster Universities Osteoarthritis (WOMAC) questionnaire. The authors found no difference in the ROM or in the clinical and radiographic results between knees that received a high-flexion cruciate-retaining or stabilized prosthesis.

The major strength of this study was that the patients acted as their own controls. Furthermore, patients were assessed clinically, radiographically, and by functional outcome by an independent observer. In addition all of the operations were carried out by one surgeon. Weaknesses of the study were that this was an Asian population of predominantly female patients and that the findings may not necessarily be transferable to other populations. In addition, when bilateral procedures are performed, it may be difficult to separate the functional outcome for one of the individual knees.

There are concerns that efforts to increase maximum flexion may negatively impact implant survival. Ranawat noted that shortening the posterior radius by removing more bone would result in instability and increased patellar and tibial stresses [16]. Other authors have shown increasing contact stresses with increasing flexion and translation of the femoro-tibial contact point, leading to potentially greater wear and earlier failure of polyethylene inserts [17, 18]. This is particularly concerning with the growing proportion of younger patients with higher functional demands receiving TKA.


This article set out to answer the question as to whether we should be using High flexion implants in patients who already have good ROM prior to their knee replacement or in the 'High flexion user'. However, the current evidence for the improved clinical benefit of using these implants in TKA over traditional implants is conflicting. More importantly none of the studies reviewed investigated the effect of these implants in patients who have good ROM (>120 degrees) prior to their surgery. Further long term studies focusing particularly on these high flexion patients are required to determine whether these patients truly benefit from these implants. The hypothesis being that those patients with good flexion prior to surgery should go on to achieve the same flexion with or without a high flexion designed implant.

There may be no doubt that theoretically High Flex design total knee replacements should help gain or increase range of motion after TKA. Methodological limitations, inconsistencies in high flexion TKA published research along with uncertain long term survivorship lead me to conclude that there is currently no established benefit in post operative knee ROM or physical function when using a High flexion implant.


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