Institute of Musculo-Skeletal Research
A prospective study of early clinical and radiological outcome of the Motus meniscal bearing total knee replacement was performed. We reviewed the first 75 consecutive prosthesis in 62 patients, implanted over a four-year period. The mean follow-up was 2.3 years. Average pre-operative Knee Society Clinical Rating score was 96 and at 2 year review 188. Average post-operative flexion at 2 years is 113 degrees (Median 119 degrees). No meniscal bearing subluxation, dislocation or breakages occurred. Radiologically there was no evidence of subsidence or osteolysis.
Our results support the continued use of this meniscal bearing knee prosthesis. However we emphasise the importance of confirming an equal flexion and extension gap without proximal joint line migration.
The potential benefits of meniscal bearing total knee prosthesis are good mobility, low contact stresses and low shear stresses on the components, thus reducing polyethylene wear and implant failure (1, 2, 3, 4, 5). However we believe that meniscal bearing prosthesis are not suitable for all patients requiring total knee replacement. We discuss the rationale for intra-operative patient selection based on equal flexion–extension gaps and report the early clinical and radiological results of the Motus Meniscal Bearing (Sliding) prosthesis.
A prospective study of clinical outcome of patients with the Motus Meniscal Bearing (Sliding) uncemented total knee replacement. The prosthesis was introduced in October 1995 and a single surgeon (MJC) performed all operations. Patient selection was performed intra-operatively. Only patients with equal flexion and extension gaps after femoral and tibial resection were selected for implantation of the Motus Sliding Meniscal prosthesis. Patients are placed on continuous passive motion (CPM) machines post-operatively and are allowed to fully weight bear immediately. All patients with a minimum 12-month follow-up were included. During the same period, 652 fixed bearing knees of a similar design were inserted.
Patients were assessed clinically and radiologically at 3, 6, 12 and 24 months post-operatively and thereafter at yearly intervals. At each review patients were scored using the Knee Society Clinical Rating System (6).
The Motus meniscal bearing total knee replacement is a posterior cruciate retaining uncemented modular prosthesis. Sintered beads covered in hydroxyapatite coat the under surface of the distal femur and the tibial plate. The femoral component (CoCrMo) is an anatomical shape and size to reproduce the natural condylar dimensions, and minimise bone resection. The trochlea groove is chamfered into the femur to reproduce the line and depth of patellar tracking. This aids knee flexion and reduces the need for lateral release. The tibial component (Ti6Al4V) has a polished upper surface and four serrated pegs on the base to aid fixation. It is designed to achieve maximal cortical coverage. The polyethylene meniscal insert has a smooth undersurface for unconstrained movement over the tibial plate and a congruent femoral articulating surface.
Over a four-year period between 1995 and 1998, 75 prosthesis were implanted in 62 patients. Mean follow-up was 2.3 years (range 1 to 4 years). There were 16 female and 46male patients with an average age of 66.8 years. 13 bilateral and 49 unilateral knee replacements were performed. 43 were in right knees and 32 in left knees. Average tourniquet time was 45 minutes. Patella replacements were performed in 19 patients in whom there was evidence of patella disease at operation. The majority of patients, 71, had osteoarthritis. Of the remainder, one had psoriatic arthropathy, one had Chondrocalcinosis, one Ollier’s disease and the last had a post-septic arthritis. Five patients had previously had a high tibial osteotomy.
The average pre-operative Knee Society Score was 96. Post-operatively the average Knee Society Score rose to 168 by 3 months and 188 at 2 years (Table 1). The average pre-operative flexion was 116 degrees. Post-operatively the average flexion was 108 degrees at 3 months and 113 degrees at 2 years (Figure 1). However the median flexion at 2 years was 119 degrees. Radiological assessment revealed no component subsidence or osteolysis. There were no meniscal bearing dislocation, subluxation or breakages
To date no patients have required revision surgery. One patient however did require a patella replacement for persistent anterior pain, which subsequently resolved. Three patients had significant arthrofibrosis, two of whom underwent arthrolysis. Other complications included two non-fatal pulmonary emboli, two deep venous thrombi, two superficial infections and two haematomas.
Table 1: CLINICAL OUTCOME
The perceived advantage of congruent, meniscal bearing knee replacements is the potential for reduced polyethylene wear while allowing unrestrained tibiofemoral movement (7, 4, 8). Meniscal bearing total knee replacements have significantly reduced upper and lower surface stresses compared with fixed-bearing components (9). The mean rate of polyethylene penetration of congruent meniscal bearings is less than that of fixed bearings and has been estimated in vitro to be as low as 0.05mm (10) to 0.01mm (4) per year.
In addition, by allowing completely unconstrained motion of the mobile bearing (as compared to central peg systems and systems where the bearings are constrained by tracks) high shear stresses in the polyethylene and at the bone/component interface are avoided. Knees undergoing replacement are diseased and their individual kinematics are impossible to forecast. As a result we believe it is essential that the mobile bearing be allowed to find its own place and range of motion to suit the knee in which it has been implanted. That is the bearing should be "self centering".
When using a meniscal bearing surface it is essential to have equal flexion and extension gaps (11). This maintains contact pressure on the bearing, helps maintain congruence and avoids problems of subluxation and dislocation. We believe that to equalise the flexion-extension gap the first cut in a total knee replacement should be the anterior femoral cut to correct femoral rotation. The distal femoral resection should follow and is determined by the thickness of the femoral prosthesis in order to maintain the normal joint line. The posterior femoral resection is determined by the size of the femoral component selected, ideally to equal the distal resection. Femoral condyle profiles differ in antero-posterior height and radii of curvature. The posterior cuts cannot therefore be equal and so the flexion space may also vary medially and laterally.
The alternative, to move the joint line to ensure a relatively equal flexion/extension space creates further problems. The extensor mechanism in particular may be compromised. Also, this enforced equality only applies at zero and ninety degrees without reference to greater or intervening flexion angles.
It has been suggested that in bicompartmental replacement, the movements of the meniscal bearing should be limited to the anteroposterior direction and that freely mobile bearings should not be used in the absence of a functioning anterior cruciate ligament (12). This has not proven to be a problem with the Motus Sliding Meniscus prosthesis, which both sacrifices the ACL and allows totally unconstrained meniscal movement.
The results of this series are comparable to outcomes achieved by other meniscal bearing prosthesis (13, 11, 5, 10). The pain relief and restoration of function as assessed both clinically and by the improvement in Knee Society Clinical Rating score is gratifying. A mean post-operative flexion of 113 degrees (median 119 degrees) compares favourably with other reports (14, 13).
The early clinical and radiological outcome achieved using the Motus Meniscal Bearing (Sliding) total knee replacement justifies its continued use. However we stress the need for appropriate intra-operative patient selection. Only those patients in who there are equal flexion and extension gaps present after completion of the femoral and tibial resections should be considered for implantation of a mobile bearing prosthesis.
1 Buechel F.F, Pappas M.J., The New Jersey Low-Contact-Stress Knee Replacement System: biomechanical rationale and review of the first 123 cemented cases. Arch Orthop Trauma Surg, 1986, 105:4, 197-204
2 Goodfellow J., O’Connor J., The mechanics of the knee and prosthesis design. J Bone Joint Surg [Br] 1978, 60-B: 358-69
3 Jordan L.R., Olivio J.L., Voorhorst P.E., Survivorship Analysis of Cementless Meniscal Bearing Total Knee Arthroplasty. Clin Orthop and Related Research No.338; 119-123, May 1997
4 Psychoyios V., Crawford R.W., O’Connor J.J., Murray D.W., Wear of congruent meniscal bearings in unicompartmental knee arthroplasty. J Bone Joint Surg [Br] Vol.80-B, No.6; 976-982, November 1998
5 Polyzoides A.J., Dendrinos G.K., Tsakonas H., The Rotaglide total knee arthroplasty. Prosthesis design and early results. J Arthroplasty, 1996 Jun, 11:4, 453-9
6 Insall J.N., Dorr L.D., Scott R.D., Scott W.N., Rationale of the Knee Society Clinical Rating System. Clin Orthop, 1989, Nov.,248:13-4
7 Murray D.W., Goodfellow J.W., O’Connor J.J., The Oxford medial unicompartmental arthroplasty. A ten-year survival study. J Bone Joint Surg [Br], Vol.80-B, No.6: 983-989, November 1998
8 Argenson J.N., O’Connor J.J., Polyethylene wear in meniscal knee replacement. A one to nine-year retrieval analysis of the Oxford knee. J Bone Joint Surg [Br], 1992 Mar, 74:2, 228-32
9 Matsuda S., White S.E., Williams V.G. 2nd, McCarthy D.S., Whiteside L.A. Contact stress analysis in meniscal bearing total knee arthroplasty. J Arthroplasty, 1998 Sep, 13: 6, 699-706
10 Tsakonas A.C., Polyzoides A.J. Reduction of polyethylene wear in a congruent meniscal knee prosthesis. Experimental and clinical studies. Acta Orthop Scand Supl, 1997 Oct, 275: 127-31
11 Buechel F.F, Pappas M.J. New Jersey low contact stress knee replacement system. Ten-year evaluation of meniscal bearings. Orthop Clin North Am, 1989 Apr, 20:2, 147-77
12 O’Connor J.J., Goodfellow J.W. Theory and practice of meniscal knee replacement: designing against wear. Proc Inst Mech Eng [H], 1996, 210:3, 217-22
13 Goodfellow J.W., O’Connor J.J. Clinical results of the Oxford knee. surface arthroplasty of the tibiofemoral joint with a meniscal bearing prosthesis. Clin Orthop, 1986 Apr, No.205: 21-42
14 Stiehl J.B., Voorhorst P.E. Total knee arthroplasty with a mobile-bearing prosthesis: comparison of retention and sacrifice of the posterior cruciate ligament in cementless implants. Am J Orthop, 1999 Apr, 28: 4, 223-8