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Early Complications After Minimally Invasive Mobile-Bearing Medial Unicompartmental Knee Arthroplasty
The Journal of Arthroplasty Vol. 24 No. 8 2009
Early Complications After Minimally Invasive Mobile-Bearing Medial Unicompartmental Knee Arthroplasty Moo-Ho Song, MD, Bu-Hwan Kim, MD, Seong-Jun Ahn, MD, Seong-Ho Yoo, MD, and Min-Soo Lee, MD
Abstract: The purpose of this article is to review the early complications of minimally invasive mobile-bearing unicompartmental knee arthroplasty by analyzing the first 100 consecutive cases. All cases were medial unicompartmental arthroplasties with a follow-up of at least 24 months (range, 24-51 months). Seven revision procedures were performed due to 2 fractures of the medial tibial plateau, 1 loosening of the femoral component, and 4 dislocations of the meniscal bearing. The early results as determined by The Knee Society scores, Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) scores and postoperative range of motion were satisfactory. However, there exists a high rate of early failures, which occurred during the initial learning curve. Therefore, surgeons must be aware that in order to minimize these complications, attention to detail is mandatory. Keywords: unicompartmental knee arthroplasty, mobile bearing, minimally invasive, complications. © 2009 Elsevier Inc. All rights reserved.
Surgical options of unicompartmental osteoarthritis include arthroscopic multiple drilling, proximal tibial osteotomy, total knee arthroplasty (TKA), or unicompartmental knee arthroplasty (UKA). Although UKA is still being debated compared to TKA, it can preserve bone stock; save anterior and posterior cruciate ligaments, patellofemoral joint, and the opposite compartment of the knee; and has the advantages of less postoperative pain, less blood loss, earlier rehabilitation, and better range of motion [1]. For these superiorities, frequency of UKA is gradually increasing together with the advancement of minimally invasive technique and surgical instruments [2]. But still, there are some concerns about the results of UKA because of a history of higher failure rates than TKA [3,4]. Our hypothesis was that the incidence of failure rates of UKA could be decreased with better surgical techniques and experiences. We are reporting the early complications and suggestions how to minimize these complications by analyzing the first 100 consecutive minimally invasive mobile-bearing UKA.
Materials and Methods This study is a retrospective clinical review of the first 100 consecutive minimally invasive mobile-bearing UKA between November 2003 and February 2006. All cases were medial unicompartmental arthroplasties with a follow-up of at least 24 months (range, 24-51 months). This study consists of 94 patients (87 women and 7 men). Of these, 6 patients underwent bilateral UKA. The average age at the time of operation was 65.7 years (range, 57-82 years); the average height was 157.4 cm (range, 138-166 cm); and the average weight was 63.4 kg (range, 43-81 kg). All patients underwent with Oxford Unicompartmental Knee Phase 3 (Biomet, South Wales, UK) under spinal anesthesia. The incision was medial parapatellar and the minimally invasive quadriceps-sparing fashion. All surgeries were performed in accordance with the surgical technique as published by the manufacturer. Finally, the wound repairment was done without any suction drainage. At postoperative day 1, the patient was permitted weight bearing as tolerated by using a walker.
Results From the Department of Orthopaedic Surgery, Daedong General Hospital, Busan, South Korea. Submitted January 31, 2009; accepted July 13, 2009. No benefits or funds were received in support of the study. Reprint requests: Moo-Ho Song, MD, Department of Orthopaedic Surgery, Daedong General Hospital, Myungryun 1-dong, Dongrae-gu, Busan 607-711, South Korea. © 2009 Elsevier Inc. All rights reserved. 0883-5403/09/2408-0027$36.00/0 doi:10.1016/j.arth.2009.07.012
1281
Surgical Parameters The average of operation time was 72 minutes (range, 50-110 minutes), and the incision length was 6.3cm (range, 5-9 cm). The suction drainage was not used, and there was no blood transfusion. Clinical Parameters The average preoperative knee score and function score were 45.9 and 41.3, respectively, which improved to 89.2
1282 The Journal of Arthroplasty Vol. 24 No. 8 December 2009 and 85.6 at 2 years. The average Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) score was 58.1 at the preoperative evaluation and was decreased to 18.5 at 2 years. The average range of motion was 126.8° (range, 105°-140°) at the preoperative evaluation and changed to 138.4° (range, 110°-150°) at 2 years. The average time taken to achieve straight-leg raising was 1.2 days (range, 1-3 days), and the average time of postoperatively 90° knee flexion was 3.6 days (range, 1-9 days). Radiological Parameters The average tibiofemoral angle with weight bearing was 2.9° of varus (range, 10° of varus to 2° of valgus) preoperatively and 3.5° of valgus (range, 2° of varus to 9° of valgus) postoperatively. Complications There were 2 fractures of medial tibial plateau (Fig. 1), 1 loosening of femoral component (Fig. 2) necessitating a revision to TKA, and 4 cases of meniscal bearing dislocations were replaced to thicker bearings. Thus, we were confronted with 7 complications (7%) and 3 (3%) revisions to TKA. However, there were no woundhealing problems, infections, or deep vein thrombosis.
Discussion A variety of procedures have been suggested for unicompartmental osteoarthritis of the knee. Among them, UKA is still being debated compared to TKA. Yet, the early studies of UKA reported that it had worse results in the long-term follow-ups and a higher revision rate than TKA. Considering the complications after UKA, Keys et al [5] suggested that the complications necessitating revision like component dislocation, loosening, or osteoarthritis of the lateral compartment appeared in the early postoperative period. Vardi and Strover [6] reported that among their 206 patients of UKA, 31 patients (15%) underwent further surgery and 13 patients (6.3%) were revised to TKA because of dislocation of polyethylene spacer, early loosening, fracture of medial tibial plateau, rapid progression of osteoarthritis in the lateral compartment, subsidence, stiffness, swelling, and infection. In our cases, there were 2 fractures of medial tibial plateau, 1 loosening of femoral component, and 4 dislocations of meniscal bearing. The fracture of medial tibial plateau after UKA has been reported relatively rare, but Lindstrand et al [7] noted that the reasons for the fracture were nonoptimal placement of component, too deeply drilled tibial peg hole, and overloading the replaced compartment as a result of the aimed undercorrection of the limb alignment. Some reported that a fracture of medial tibial plateau could be healed by conservative treatment, but Brumby et al [8] reported that all 4 cases of the fracture detected in the early postoperative period failed with conservative treatment and were revised to TKA. In our 2
Fig. 1. (A) Radiograph showing fracture of the medial tibial plateau after 4 weeks of UKA. (B) Revision with TKA.
cases with conservative treatment, no fracture union was found. During operation, we found the bone mass and quality of proximal tibia inadequate for bone union and the fracture fragment too small to get rigid fixation, which lead us to TKA revision.
Mobile-Bearing Medial Unicompartmental Knee Arthroplasty Song et al
Fig. 2. (A) Radiograph showing loosening of the femoral component after 4 months of UKA. (B) Revision with TKA.
Some ways to avoid a fracture of medial tibial plateau are as follows: make a conservative initial tibial resection to preserve bone stock, make adequate slot for the keel of tibial component, restore valgus alignment to avoid excessive over loading to the medial compartment, preserve posterior cortex during vertical tibial.
1283
saw cut, use light hammering, and permit postoperative weight bearing cautiously, especially with elderly or osteoporotic patients. It is difficult to know what the exact cause of loosening of the femoral component is, but Riebel et al [9], analyzing 12 cases of loosening the femoral component and 2 cases of dislocation of femoral component of their 100 UKA with Howmedica PCA unicompartmental arthroplasty, noted that the design of the femoral implant itself played role in the increased rate of early clinical loosening rate. In our case, there was 1 femoral component loosening that was revised into TKA. Intraoperatively, we have found that the contact status of the cement between the component and the femur was not adequate enough and the bony resection in the posterior femoral condyle was relatively large, which caused the instability between the component and the femur. In addition, only 1 peg in the component could not hold tightly enough against the rotational force. We concluded that these 3 factors were possible causes of this complication. Our suggestions to avoid this complication are as follows: saw blade must be parallel to the underside of the saw block, do not make a gap between the posterior femoral condyle and the component, make a lot of cement holes in the femoral condyle before implant the component, and the cement must be liquid enough during the working phase to penetrate the cancellous bone achieving micro-interlocking. Meniscal bearing dislocation was introduced into surgery with the invention of mobile-bearing knee arthroplasty [10]. Lewold et al [11], in evaluating the Swedish Registry, found that the risk was 2.3% and that it was the most common cause of failure in Oxford UKA phase 1 and 2 (16 of 50 revisions). Price [12] found a difference in the rates of dislocation between Oxford UKA phase 1 (2.5%) and phase 2 (0.5%). In the phase 3 cohort [13], it was 0.2 %. However, bearing dislocation is a potential complication in Asian populations who need to sit on the floor cross-legged, kneeling, or squatting. There were 4 bearing dislocations, 2 anterior and 2 posterior in our series. All the bearings were exchanged with the thicker ones or the anatomical bearing. The possible cause of one anterior dislocation was retained osteophytes in the posterior femoral condyle. In other 3 cases, elongation of the medial collateral ligament (MCL) was found during the revision. There was no recurrent dislocation because there were no serious mismatches between gaps or alignment, so revision to TKA was not necessary. The main causes of bearing dislocations were inadequate entrapment of the bearing due to inequality of the flexion and extension gaps, delayed elongation of the MCL over time, and impingement caused by the osteophytes from the back of the femoral condyle. The ways to minimize bearing dislocations are as follows: achieve proper soft tissue balancing, locate components exactly to avoid malposition, remove all possible causes of impingement, protect MCL during the surgery, and use the anatomical bearing.
1284 The Journal of Arthroplasty Vol. 24 No. 8 December 2009 It has been reported that the result of UKA is related to operative technique or a learning curve. Lindstrand et al [7] found, analyzing 123 UKA in a multicenter study composed of 4 other hospitals, that there were the 8 revisions, 7 of which occurred among the first 10 arthroplasties in each hospital, and revision was related to operative technique. So, they believed that in almost every surgeon's hands, a given UKA would give nonoptimal early results during the learning curve, and at least 10 to 15 UKA per year were probably necessary to maintain a reasonable expertise in these procedures. In our experience, all 3 cases of major complication necessitating revision to TKA occurred during the first 30 cases, and after that, no more severe complications have occurred. This means that these complications may be related to the lack of operative technique and experience. So, if a surgeon gets the exact operative techniques in the early learning period and follows up patients carefully to avoid complications especially in elderly patients and osteoporotic patients, the incidence of these complications will decrease.
References 1. Berger RA, Nedeff DD, Barden RM, et al. Unicompartmental knee arthroplasty: clinical experience at 6- to 10-year followup. Clin Orthop Relat Res 1999;367:50. 2. Riddle DL, Jiranek WA, McGlynn FJ. Yearly incidence of unicompartmental knee arthroplasty in the united states. J Arthroplasty 2008;23:408. 3. Newman JH. UKR—time for caution? Knee 2004;11:337.
4. Vince KG, Cyran LT. Unicompartmental knee arthroplasty: new indication, more complication? J Arthroplasty 2004; 19:9. 5. Keys GW, Ul-Abiddin Z, Toh EM. Analysis of first forty Oxford medial unicompartmental knee replacement from a small district hospital in UK. Knee 2004;11:375. 6. Vardi G, Strover AE. Early complications of unicompartmental knee replacement: the Droitwich experience. Knee 2004;11:389. 7. Lindstrand A, Stenstrom A, Ryd L, et al. The introduction period of unicompartmental knee arthroplasty is critical: a clinical, clinical multicentered, and radiostereometric study of 251 Duracon unicompartmental knee arthroplasties. J Arthroplasty 2000;15:608. 8. Brumby SA, Carrington R, Zayontz S, et al. Tibial plateau stress fracture: a complication of unicompartmental knee arthroplasty using 4 guide pinholes. J Arthroplasty 2003;18: 809. 9. Riebel GD, Werner FW, Ayres DC, et al. Early failure of the femoral component in unicompartmental knee arthroplasty. J Arthroplasty 1995;10:615. 10. Goodfellow JW, O' Connor J. The mechanics of the knee and prosthesis design. J Bone Joint Surg [Br] 1978;60:358. 11. Lewold S, Goodman S, Knutson K, et al. Oxford meniscal bearing knee versus the Marmor knee in unicompartmental arthroplasty for arthrosis. A Swedish multicenter survival study. J Arthroplasty 1995;10:722. 12. Price AJ. Medial meniscal bearing unicompartmental arthroplasty : wear, mechanics and clinical outcome. DPhil Thesis, University of Oxford, 2003. 13. Pandit H, Jenkins C, Baker K, et al. The Oxford medial unicompartmental knee replacing using a minimally invasive approach. J Bone Joint Surg [Br] 2006;88B:54.
Early Complications After Minimally Invasive Mobile-Bearing Medial Unicompartmental Knee Arthroplasty Moo-Ho Song, MD, Bu-Hwan Kim, MD, Seong-Jun Ahn, MD, Seong-Ho Yoo, MD, and Min-Soo Lee, MD
Abstract: The purpose of this article is to review the early complications of minimally invasive mobile-bearing unicompartmental knee arthroplasty by analyzing the first 100 consecutive cases. All cases were medial unicompartmental arthroplasties with a follow-up of at least 24 months (range, 24-51 months). Seven revision procedures were performed due to 2 fractures of the medial tibial plateau, 1 loosening of the femoral component, and 4 dislocations of the meniscal bearing. The early results as determined by The Knee Society scores, Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) scores and postoperative range of motion were satisfactory. However, there exists a high rate of early failures, which occurred during the initial learning curve. Therefore, surgeons must be aware that in order to minimize these complications, attention to detail is mandatory. Keywords: unicompartmental knee arthroplasty, mobile bearing, minimally invasive, complications. © 2009 Elsevier Inc. All rights reserved.
Surgical options of unicompartmental osteoarthritis include arthroscopic multiple drilling, proximal tibial osteotomy, total knee arthroplasty (TKA), or unicompartmental knee arthroplasty (UKA). Although UKA is still being debated compared to TKA, it can preserve bone stock; save anterior and posterior cruciate ligaments, patellofemoral joint, and the opposite compartment of the knee; and has the advantages of less postoperative pain, less blood loss, earlier rehabilitation, and better range of motion [1]. For these superiorities, frequency of UKA is gradually increasing together with the advancement of minimally invasive technique and surgical instruments [2]. But still, there are some concerns about the results of UKA because of a history of higher failure rates than TKA [3,4]. Our hypothesis was that the incidence of failure rates of UKA could be decreased with better surgical techniques and experiences. We are reporting the early complications and suggestions how to minimize these complications by analyzing the first 100 consecutive minimally invasive mobile-bearing UKA.
Materials and Methods This study is a retrospective clinical review of the first 100 consecutive minimally invasive mobile-bearing UKA between November 2003 and February 2006. All cases were medial unicompartmental arthroplasties with a follow-up of at least 24 months (range, 24-51 months). This study consists of 94 patients (87 women and 7 men). Of these, 6 patients underwent bilateral UKA. The average age at the time of operation was 65.7 years (range, 57-82 years); the average height was 157.4 cm (range, 138-166 cm); and the average weight was 63.4 kg (range, 43-81 kg). All patients underwent with Oxford Unicompartmental Knee Phase 3 (Biomet, South Wales, UK) under spinal anesthesia. The incision was medial parapatellar and the minimally invasive quadriceps-sparing fashion. All surgeries were performed in accordance with the surgical technique as published by the manufacturer. Finally, the wound repairment was done without any suction drainage. At postoperative day 1, the patient was permitted weight bearing as tolerated by using a walker.
Results From the Department of Orthopaedic Surgery, Daedong General Hospital, Busan, South Korea. Submitted January 31, 2009; accepted July 13, 2009. No benefits or funds were received in support of the study. Reprint requests: Moo-Ho Song, MD, Department of Orthopaedic Surgery, Daedong General Hospital, Myungryun 1-dong, Dongrae-gu, Busan 607-711, South Korea. © 2009 Elsevier Inc. All rights reserved. 0883-5403/09/2408-0027$36.00/0 doi:10.1016/j.arth.2009.07.012
1281
Surgical Parameters The average of operation time was 72 minutes (range, 50-110 minutes), and the incision length was 6.3cm (range, 5-9 cm). The suction drainage was not used, and there was no blood transfusion. Clinical Parameters The average preoperative knee score and function score were 45.9 and 41.3, respectively, which improved to 89.2
1282 The Journal of Arthroplasty Vol. 24 No. 8 December 2009 and 85.6 at 2 years. The average Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) score was 58.1 at the preoperative evaluation and was decreased to 18.5 at 2 years. The average range of motion was 126.8° (range, 105°-140°) at the preoperative evaluation and changed to 138.4° (range, 110°-150°) at 2 years. The average time taken to achieve straight-leg raising was 1.2 days (range, 1-3 days), and the average time of postoperatively 90° knee flexion was 3.6 days (range, 1-9 days). Radiological Parameters The average tibiofemoral angle with weight bearing was 2.9° of varus (range, 10° of varus to 2° of valgus) preoperatively and 3.5° of valgus (range, 2° of varus to 9° of valgus) postoperatively. Complications There were 2 fractures of medial tibial plateau (Fig. 1), 1 loosening of femoral component (Fig. 2) necessitating a revision to TKA, and 4 cases of meniscal bearing dislocations were replaced to thicker bearings. Thus, we were confronted with 7 complications (7%) and 3 (3%) revisions to TKA. However, there were no woundhealing problems, infections, or deep vein thrombosis.
Discussion A variety of procedures have been suggested for unicompartmental osteoarthritis of the knee. Among them, UKA is still being debated compared to TKA. Yet, the early studies of UKA reported that it had worse results in the long-term follow-ups and a higher revision rate than TKA. Considering the complications after UKA, Keys et al [5] suggested that the complications necessitating revision like component dislocation, loosening, or osteoarthritis of the lateral compartment appeared in the early postoperative period. Vardi and Strover [6] reported that among their 206 patients of UKA, 31 patients (15%) underwent further surgery and 13 patients (6.3%) were revised to TKA because of dislocation of polyethylene spacer, early loosening, fracture of medial tibial plateau, rapid progression of osteoarthritis in the lateral compartment, subsidence, stiffness, swelling, and infection. In our cases, there were 2 fractures of medial tibial plateau, 1 loosening of femoral component, and 4 dislocations of meniscal bearing. The fracture of medial tibial plateau after UKA has been reported relatively rare, but Lindstrand et al [7] noted that the reasons for the fracture were nonoptimal placement of component, too deeply drilled tibial peg hole, and overloading the replaced compartment as a result of the aimed undercorrection of the limb alignment. Some reported that a fracture of medial tibial plateau could be healed by conservative treatment, but Brumby et al [8] reported that all 4 cases of the fracture detected in the early postoperative period failed with conservative treatment and were revised to TKA. In our 2
Fig. 1. (A) Radiograph showing fracture of the medial tibial plateau after 4 weeks of UKA. (B) Revision with TKA.
cases with conservative treatment, no fracture union was found. During operation, we found the bone mass and quality of proximal tibia inadequate for bone union and the fracture fragment too small to get rigid fixation, which lead us to TKA revision.
Mobile-Bearing Medial Unicompartmental Knee Arthroplasty Song et al
Fig. 2. (A) Radiograph showing loosening of the femoral component after 4 months of UKA. (B) Revision with TKA.
Some ways to avoid a fracture of medial tibial plateau are as follows: make a conservative initial tibial resection to preserve bone stock, make adequate slot for the keel of tibial component, restore valgus alignment to avoid excessive over loading to the medial compartment, preserve posterior cortex during vertical tibial.
1283
saw cut, use light hammering, and permit postoperative weight bearing cautiously, especially with elderly or osteoporotic patients. It is difficult to know what the exact cause of loosening of the femoral component is, but Riebel et al [9], analyzing 12 cases of loosening the femoral component and 2 cases of dislocation of femoral component of their 100 UKA with Howmedica PCA unicompartmental arthroplasty, noted that the design of the femoral implant itself played role in the increased rate of early clinical loosening rate. In our case, there was 1 femoral component loosening that was revised into TKA. Intraoperatively, we have found that the contact status of the cement between the component and the femur was not adequate enough and the bony resection in the posterior femoral condyle was relatively large, which caused the instability between the component and the femur. In addition, only 1 peg in the component could not hold tightly enough against the rotational force. We concluded that these 3 factors were possible causes of this complication. Our suggestions to avoid this complication are as follows: saw blade must be parallel to the underside of the saw block, do not make a gap between the posterior femoral condyle and the component, make a lot of cement holes in the femoral condyle before implant the component, and the cement must be liquid enough during the working phase to penetrate the cancellous bone achieving micro-interlocking. Meniscal bearing dislocation was introduced into surgery with the invention of mobile-bearing knee arthroplasty [10]. Lewold et al [11], in evaluating the Swedish Registry, found that the risk was 2.3% and that it was the most common cause of failure in Oxford UKA phase 1 and 2 (16 of 50 revisions). Price [12] found a difference in the rates of dislocation between Oxford UKA phase 1 (2.5%) and phase 2 (0.5%). In the phase 3 cohort [13], it was 0.2 %. However, bearing dislocation is a potential complication in Asian populations who need to sit on the floor cross-legged, kneeling, or squatting. There were 4 bearing dislocations, 2 anterior and 2 posterior in our series. All the bearings were exchanged with the thicker ones or the anatomical bearing. The possible cause of one anterior dislocation was retained osteophytes in the posterior femoral condyle. In other 3 cases, elongation of the medial collateral ligament (MCL) was found during the revision. There was no recurrent dislocation because there were no serious mismatches between gaps or alignment, so revision to TKA was not necessary. The main causes of bearing dislocations were inadequate entrapment of the bearing due to inequality of the flexion and extension gaps, delayed elongation of the MCL over time, and impingement caused by the osteophytes from the back of the femoral condyle. The ways to minimize bearing dislocations are as follows: achieve proper soft tissue balancing, locate components exactly to avoid malposition, remove all possible causes of impingement, protect MCL during the surgery, and use the anatomical bearing.
1284 The Journal of Arthroplasty Vol. 24 No. 8 December 2009 It has been reported that the result of UKA is related to operative technique or a learning curve. Lindstrand et al [7] found, analyzing 123 UKA in a multicenter study composed of 4 other hospitals, that there were the 8 revisions, 7 of which occurred among the first 10 arthroplasties in each hospital, and revision was related to operative technique. So, they believed that in almost every surgeon's hands, a given UKA would give nonoptimal early results during the learning curve, and at least 10 to 15 UKA per year were probably necessary to maintain a reasonable expertise in these procedures. In our experience, all 3 cases of major complication necessitating revision to TKA occurred during the first 30 cases, and after that, no more severe complications have occurred. This means that these complications may be related to the lack of operative technique and experience. So, if a surgeon gets the exact operative techniques in the early learning period and follows up patients carefully to avoid complications especially in elderly patients and osteoporotic patients, the incidence of these complications will decrease.
References 1. Berger RA, Nedeff DD, Barden RM, et al. Unicompartmental knee arthroplasty: clinical experience at 6- to 10-year followup. Clin Orthop Relat Res 1999;367:50. 2. Riddle DL, Jiranek WA, McGlynn FJ. Yearly incidence of unicompartmental knee arthroplasty in the united states. J Arthroplasty 2008;23:408. 3. Newman JH. UKR—time for caution? Knee 2004;11:337.
4. Vince KG, Cyran LT. Unicompartmental knee arthroplasty: new indication, more complication? J Arthroplasty 2004; 19:9. 5. Keys GW, Ul-Abiddin Z, Toh EM. Analysis of first forty Oxford medial unicompartmental knee replacement from a small district hospital in UK. Knee 2004;11:375. 6. Vardi G, Strover AE. Early complications of unicompartmental knee replacement: the Droitwich experience. Knee 2004;11:389. 7. Lindstrand A, Stenstrom A, Ryd L, et al. The introduction period of unicompartmental knee arthroplasty is critical: a clinical, clinical multicentered, and radiostereometric study of 251 Duracon unicompartmental knee arthroplasties. J Arthroplasty 2000;15:608. 8. Brumby SA, Carrington R, Zayontz S, et al. Tibial plateau stress fracture: a complication of unicompartmental knee arthroplasty using 4 guide pinholes. J Arthroplasty 2003;18: 809. 9. Riebel GD, Werner FW, Ayres DC, et al. Early failure of the femoral component in unicompartmental knee arthroplasty. J Arthroplasty 1995;10:615. 10. Goodfellow JW, O' Connor J. The mechanics of the knee and prosthesis design. J Bone Joint Surg [Br] 1978;60:358. 11. Lewold S, Goodman S, Knutson K, et al. Oxford meniscal bearing knee versus the Marmor knee in unicompartmental arthroplasty for arthrosis. A Swedish multicenter survival study. J Arthroplasty 1995;10:722. 12. Price AJ. Medial meniscal bearing unicompartmental arthroplasty : wear, mechanics and clinical outcome. DPhil Thesis, University of Oxford, 2003. 13. Pandit H, Jenkins C, Baker K, et al. The Oxford medial unicompartmental knee replacing using a minimally invasive approach. J Bone Joint Surg [Br] 2006;88B:54.