BACKGROUND
==========

Increasing efforts have been made over the past years to improve the flexion properties of TKA´s by refining their design. The basic principle in doing so has been to reduce the back side radius of the condyle of the femoral component, which, in combination with partial posterior coupling, has achieved a maximum flexion angle of over 120° \[[@R1]\]. Although this means that resection of more bone from the posterior femoral condyle is unavoidable, this permits a range of motion extending to deep kneeling and squatting.

Due to demographic changes increasingly young patients require TKA, and this means that the expectations of surgery (e.g. flexion over 120°) are also increasing. It is no longer enough to ensure freedom from pain and to allow everyday activities. In most cases patients expect a free range of motion which is often promised by the advertisement of high flex TKA´s \[[@R2]\].

A knee flexion of 90º to 100º is enough to allow most activities of daily living for elderly people in western countries. Climbing up and down the stairs and sitting on a chair requires 90 to 120º. This range of motion can be achieved with traditional TKA designs whose maximal flexion angle has been reported to range between 100 and 110º \[[@R3]\]. Apart from being influenced by the condition of the patient and surgical technique, the final outcome of a TKA, depends also on the implant design. Therefore, implant manufacturers have attempted to design TKAs that better accommodate knee mechanics in high flexion up to 155° \[[@R4],[@R5]\] to allow patients activities such as squatting and sitting cross-legged which require knee flexion of 110 to 130º \[[@R3]\].

Recently there have been published similar studies evaluating high flexion knee implants. Kim *et al.*compared the results of a standard high flexion total knee replacement versus a gender specific high flexion implant type \[[@R6]\]. They found no significant differences between the two groups with regard to the clinical and radiological results, patient satisfaction or complication rate. Another study by Kim *et al.*showed after a minimum duration of follow-up of two years, that there was no difference in range of motion or clinical and radiographic results between knees that had received high-flexion posterior cruciate-retaining total knee prosthesis and those that had received a high-flexion posterior cruciate-substituting total knee prosthesis \[[@R7]\].

We want to present the results of 75 NexGen® CR-Flex Mobile TKA´s performed by a single surgeon (prosthetic system with high flexion capacity, Zimmer) in a 100% mid-term follow up of 5 years and try to compare the functional outcome with the recent literature on TKA´s, especially on traditional TKA´s

METHODS
=======

Study Design
------------

Between January 2005 and January 2006 75 NexGen® Cr-Flex Mobile (Zimmer) TKA´s were performed by the author due to primary osteoarthritis. None of the patients received a bilateral TKA (Table **[1](#T1){ref-type="table"}**).

The study protocol of this retrospective study, including the consent forms, was approved by the institutional review board of the University hospital of Marburg, Germany. A detailed informed consent form was signed by each patient, and all information was kept confidential.

Patients received preoperative single shot antibiotic prophylaxis with Cefuroxim 1.5g (if renal function allowed). A tourniquet was used after exsanguination using an Esmarch bandage. All patients received thromboprophylaxis with Mono-Embolex 0.3 once daily.

All TKA procedures were performed by the author (single surgeon) using the anterior Payr-approach with medial arthrotomy. Soft tissues were released in a stepwise manner to achieve ligamentous balance in extension.

Thus, flexion and extension gaps were approximately equal. Sufficient laxity was achieved to enable full extension and flexion and anterior translation, but not too loose to cause abnormal AP motion with resulting impingement or bearing spin out. Tensioning devices were not routinely used. The proximal tibia was resected with the anterior cruciate ligament sacrificed. The PCL was preserved with a bone block.

Both the femoral and the tibial component were completely cemented (Palacos®). Primary patellar resurfacing was not performed in any of the patients.

Hospital Care and Rehabilitation
--------------------------------

All patients were inpatients and were observed postoperatively for one day on an intermediate care ward. Physiotherapy was started on the day after surgery with walking exercises on two forearm crutches with maximum weight-bearing and exercise on CPM (continous passive motion). Lymph drainage was added on the second postoperative day.

### Investigation Parameters

The follow up was done by the author. ROM, KSS \[[@R8]\] and radiological findings (knee in 3 planes) were evaluated before and 5 years after surgery. Performance of the replaced knee and overall function were assessed using the Knee Society knee and functional scores (maximum score 200, 100 each). The scoring system combines a relatively objective Knee Score that is based on the clinical parameters and a functional Score based on how the patient perceives that the knee functions with specific activities. The maximum Knee Score is 100 points and the maximum functional Score is 100 points (Overall 200 points).

The range of motion especially the maximum passive flexion were assessed using a goniometer with the arms aligned along the long axes of the femur and tibia on the lateral side of the knee joint.

ROM was assessed as primary outcome whereas KSS as secondary outcome. Radiographic assessment was done to identify possible complications like radiolucent lines at the bone cement interface, patella tracking, limb alignment and positioning of the implants. It wasn't done as a third outcome measurement.

Radiographic assessment consists of a standing scanogram of the leg and lateral radiography to assess tibiofemoral alignment and component positioning. The Merchant's view was used to assess patellar tracking. The axial patellofemoral position was defined as central (\<5° tilted) or as having medial tilt (\>5° tilted, with the medial side depressed) or lateral tilt (\>5° tilted, with the lateral side depressed) as described by Bindelglass and Vince \[[@R9]\].

Statistical Methods
-------------------

Data were collated and interpreted by the author to minimise intra-observer error. Differences were compared using t-test. A p value of \<0.05 was considered significant. The scores and ROM´s followed a normal distribution.

RESULTS
=======

The follow-up period was 5 years. 75 (100%) knees were followed up clinically and radiological.

Scores and Range of Motion (Table [2](#T2){ref-type="table"})
-------------------------------------------------------------

The Range of Motion improved continuously, so a mean flexion of 122° (SD: 10.6°) and a flexion contracture of 3.9° (SD: 6.6°) was achieved after five years. The mean preoperative KSS was 99 (SD: 12; max. 200). 5 years postoperative an improvement was evident. The mean KSS at that time was 167 (SD: 21; max. 200).

Over the period of 5 there was a highly significant improvement in comparison to the preoperative status (p\<0.005 for KSS and ROM).

Radiographic Assessment
-----------------------

The tibiofemoral angle was 5--8° valgus in 26 knee joints, 0--4° valgus in 15 joints, and 0--8° varus in 34 knee joints. The mean posterior slope of the tibia was 6.4° (SD: 1.8°). None of the joints showed signs of aseptic loosening at follow-up. The patella was centrally positioned in 67 cases (\<5° tilt), showed medial tilt in no cases, lateral tilt without subluxation in 6 cases, and lateral tilt with subluxation in 2 cases (\>5° lateral tilt).

Complications
-------------

3 patients developed postoperative complications requiring treatment. 2 of these patients developed deep vein thrombosis and had to be treated with anticoagulants for 3 months. The 1 remaining patient demonstrated in the first postoperative week unsatisfactory ROM (\<60° flexion), so that a mobilisation under general anaesthesia in combination with a peridural catheter for analgesia was done. Implant-specific complications like aseptic lossening, dislocation of the polyethylene insert, etc. were not seen over the 5 year follow up. 8 patients with lateral tilt of the patella and subluxation underwent revision surgery during the first postoperative year with patellar resurfacing, which improved their condition.

DISCUSSION
==========

The aim of a TKA design is to restore function and normal knee kinematics so as to minimise the risk of implant failure, particularly in younger patients who are subject to higher revision rates \[[@R10]\]. Mobile bearings are superior to fixed bearings because of the reduced shear stress within the polyethylene and improved wear properties.

The aim of the present study was to evaluate a new multi-radius designed TKA (Zimmer Warsaw, Indiana, USA NexGen Cr-Flex Mobile) in mid-term follow up with regard to the functional outcome and to compare the results to the recent literature, especially with the results of traditional TKA´s.

The main difference to traditional TKA´s is on the one hand the fact that the NexGen CR-Flex Mobile has a groove on the anterior sliding surfaces that provides more room for the patellar tendon on increasing flexion. And on the other hand, that the rotational center of the polyethylene inlay is located on the medial tibial plateau and not centrally as with most other TKA´s. In addition, the radius of the posterior femoral condyle is so that an optimal tibiofemoral contact area can be ensured even on deeper flexion.

But the success of a TKA depends critically on its service life and also on the reduction of pain and restoration of function. As a result of the changed demands that are made of a TKA nowadays, systems with extended flexion have come on the market.

Postoperative knee flexion after total knee arthroplasty (TKA) is important and correlates closely with patient satisfaction and functional level. Many factors may affect postoperative range of flexion after TKA but as shown by Kawamura et Bourne \[[@R11]\] the preoperative range of flexion and the preoperative tibiofemoral varus/valgus angles werethe factors that affected postoperative range of flexion. The tilt angles of the patella and the patellar button may also be factors that affect the postoperative range of flexion. Other factors like BMI (Body Mass Index), surgical approach, age of the patient or patella tilt did not show any correlation with the achievable flexion angle.

Previous studies have produced conflicting findings. For example: Huang *et al.*\[[@R5]\] found that the mean flexion in patients with high-flexion prosthesis was approximately 10° greater than in those with a standard posterior stabilised implant. Laskin \[[@R12]\] has also previously indicated similar findings. However, some authors \[[@R13],[@R14]\] have not noted increased flexion when using high-flexion rather than traditional prostheses.

The KSS improved from a mean of 99 before surgery to a mean of 167 at final follow up. Our scores were similar to those of other traditional TKA´s. The maximum flexion of 122° in mean was achieved by the patients already after 2 years \[[@R12]\] and is similar to recent studies \[[@R6],[@R7],[@R13]-[@R24]\]. Table **[3](#T3){ref-type="table"}** gives an overview about recently published studies evaluating traditional TKA´s and high flexion TKA´s with main focus on the flexion ability.

But, in addition to pain reduction and restoration of functionality, survivorship is also a decisive consideration in the success of TKA´s. Thanks to its extended posterior condyle radius which has been broadened all around, the NexGenCr-Flex Mobile System offer a larger contact surface on deep bending and therefore spreads the contact stress over a large area. This may guarantee optimum contact area and counteracts a high degree of polyethylene wear which may lead to an increased survivorship. But this needs to be proven in a long term follow up over 10 to 15 years.

The present study has some limitations. First, the use of KSS addresses functionality in a very limited way. The WOMAC and SF-36 are more responsive measures of the outcomes of TKA´s \[[@R25]\]. Secondly, the mid term follow up makes it difficult to draw conclusions regarding the theoretical advantage of the high flex knee replacement system regarding the lifespan. Despite these problems, we believe that we were able to give accurate information after careful assessment of the performance of this high flexion TKA.

Despite positive results in the first 5 postoperative years, the NexGen Cr Flex mobile TKA shows no significant advantages over time compared to traditional TKA´s or other high flexion TKA´s especially with regard to function and range of motion.

So called high flexion TKA´s have to proof their superiority in the long term follow up (10 to 15 years), with the focus on the degree of polyethylene wear and the possible consequence of aseptic loosening.

###### 

Patient Characteristics Preoperative (All Patients Received a Total Knee Arthroplasty Only on One Side)

                        (Mean Values)
  --------------------- -----------------
  KSS                   99 (SD: 12)
  Maximum Flexion       82° (SD: 6.5°)
  Flexion contracture   8.9° (SD: 9.1°)
  BMI                   26.6 (SD: 2.8)
  Age                   65.4 (SD: 8.1)
  Gender                m/w = 29/46
  Varus/Valgus          54/21
  Lost to follow Up     None

###### 

Comparison ROM and Knee Society Knee and Function Score Preoperative and at Final Follow Up

                                                NexGen Cr Flex Mobile
  --------------------------------------------- -----------------------
  **Maximum Flexion**                           
    Preop                                       82° (SD: 6.5°)
    Final                                       122° (SD: 10.6°)
  **Flexion Contracture**                       
    Preop                                       8.9° (SD: 9.1°)
    Final                                       3.9° (SD: 6.6°)
  **Knee Society Knee and Functional Scores**   
    Preop                                       99 (SD: 12)
    Final                                       167 (SD: 21)

###### 

Overview About the Recent Literature Especially with Regard to Function/Flexion Ability After TKA

  --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
  Author                       Study Design           Journal                      Number of Patients         TKA                                  Average Follow Up   Average Flexion Angle
  ---------------------------- ---------------------- ---------------------------- -------------------------- ------------------------------------ ------------------- -----------------------
  Huang HT,\                   Matched pair control   J Arthroplasty 2005          25 patients                High Flex *vs* Traditional PS        2.4 years           138° *vs* 126°
  Su JY *et al.*                                                                                                                                                       

  Laskin RS                    Cohort study           Orthopedics 2007             80 Patients                Traditional PS *vs* High Flex PS     2 years             118° *vs* 133°

  Kim YH,\                     RCT                    J Bone Joint Surg Br. 2010   138 patients (bilateral)   High Flex CR *vs* Gender Flex        3.25 years          126° *vs* 124°
  Choi Y *et al.*                                                                                                                                                      

  Kim YH,\                     RCT                    J Bone Joint Surg Am. 2009   250 patients (bilateral)   High Flex CR *vs* High Flex PS       2.3 years           133° *vs* 135°
  Choi Y *et al.*                                                                                                                                                      

  Kim YH,\                     RCT                    J Bone Joint Surg Am. 2005   50 patients (bilateral)    Traditional LPS *vs* LPS Flex        2.1 years           135.8° *vs* 138.6°
  Sohn KS, Kim JS                                                                                                                                                      

  Seon JK,\                    RCT                    Orthopedics 2005             100 patients               Traditional CR *vs* High Flex        2 years             130.7° *vs* 128.5°
  Song EK *et al.*                                                                                                                                                     

  Aglietti P,\                 Follow Up              J Arthroplasty. 1999         60 patients                Traditional LPS *vs* LPS Flex        10 years            106°
  Buzzi R *et al.*                                                                                                                                                     

  Gill GS,\                    Follow Up              Am J Knee Surg. 2001         223 patients               Traditional TKA CR                   16.8 years          n.a.
  Joshi AB                                                                                                                                                             

  Vince KG,\                   Follow Up              J Bone Joint Surg Br. 1989   58 patients                Traditional TKA CR                   10-12 years         91.2°
  Insall JN *et al.*                                                                                                                                                   

  Nutton RW,\                  RCT                    J Bone Joint Surg Br. 2008   56 patients                Traditional PS *vs* High Flex PS     1 year              121° *vs* 127°
  van der Linden ML *et al.*                                                                                                                                           

  Seon JK, Park SJ *et al.*    Prospective            J Bone Joint Surg Am. 2009   100 patients               Traditional CR *vs* High Flex CR     2 years             134.3° *vs* 135.3°

  Bhan S,\                     Prospective            J Bone Joint Surg Am. 2005   32 patients                Traditional CR                       6 years             106.9°
  Malhotra R *et al.*                                                                                                                                                  

  Chaudhary R,\                RCT                    J Bone Joint Surg Am. 2008   100 patients               Traditional CR *vs* Traditional PS   2 years             105.9° *vs* 105.8°
  Beaupré LA *et al.*                                                                                                                                                  
  --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
