Orthopedics

Case Report 

Late Mobile-Bearing Dislocation in Unicompartmental Knee Arthroplasty

Moo-Ho Song, MD, PhD; Kyung-Taek Kim, MD; Youn-Soo Hwang, MD; Jin-Wan Kim, MD; Tae-Woong Eom, MD; Jeong-Hoon Chae, MD

Abstract

The authors present 2 cases of late mobile-bearing dislocation after unicompartmental knee arthroplasty with long-term follow-up. Patients had anterior bearing dislocation more than 10 years after primary unicompartmental knee arthroplasty. Retrieved mobile bearings showed severe erosion on the posterior lip, suggesting frequent deep knee flexion as the cause of wear. Bearing dislocation is known to occur in the early postoperative phase. However, for Asians with a lifestyle that requires high knee flexion, dislocation might occur in the late postoperative phase. Such late bearing dislocation has a mechanism different from those reported in previous studies. Loss of bump in the posterior lip was found to be the cause. After late bearing dislocation, conversion to total knee arthroplasty is unnecessary because a simple bearing exchange will suffice. [Orthopedics. 2019; 42(1):e124–e127.]

Abstract

The authors present 2 cases of late mobile-bearing dislocation after unicompartmental knee arthroplasty with long-term follow-up. Patients had anterior bearing dislocation more than 10 years after primary unicompartmental knee arthroplasty. Retrieved mobile bearings showed severe erosion on the posterior lip, suggesting frequent deep knee flexion as the cause of wear. Bearing dislocation is known to occur in the early postoperative phase. However, for Asians with a lifestyle that requires high knee flexion, dislocation might occur in the late postoperative phase. Such late bearing dislocation has a mechanism different from those reported in previous studies. Loss of bump in the posterior lip was found to be the cause. After late bearing dislocation, conversion to total knee arthroplasty is unnecessary because a simple bearing exchange will suffice. [Orthopedics. 2019; 42(1):e124–e127.]

The frequency of unicompartmental knee arthroplasty (UKA) is on the rise as a result of its advantages, including better functional ability, shorter recovery time, and lower mortality and morbidity compared with total knee arthroplasty.1 There are 2 types of UKA: fixed bearing and mobile bearing. Each type has advantages and disadvantages.2 Mobile-bearing UKA has a fully congruous bearing surface that allows lower contact stress, resulting in reduced polyethylene wear with better outcome in long-term follow-up.3–5 However, unexpected bearing dislocation is one of the most common causes of reoperation with the use of mobile-bearing UKA, remaining a challenge to knee surgeons.6

Most bearing dislocations occur in the early postoperative period. Lewold et al7 reported that most dislocations occur within the first 2 years postoperatively. Lisowski et al8 showed that the bearing dislocation of Oxford (Biomet, Warsaw, Indiana) UKA primarily occurs shortly after implantation, probably due to technical error during surgery. Late bearing dislocation after long-term follow-up of Oxford UKA has not been reported. The authors present 2 cases of late meniscal bearing dislocation that occurred more than 10 years after primary Oxford UKA.

Case Reports

Patient 1

A 68-year-old woman (height, 156 cm; weight, 68 kg; body mass index, 28 kg/m2) had medial osteoarthritis of the left knee. She underwent Oxford phase III UKA in March 2006. With a minimally invasive approach, femoral and tibial components were fixed using bone cement, and a 5-mm–thick polyethylene bearing was inserted. Postoperative radiographs showed good alignment of femoral and tibial components (Figure 1A). She was satisfied with her postoperative condition, having normal gait and being free of pain with knee range of motion of 0° to 140°.

Postoperative anteroposterior radiograph showing good alignment of the femoral and tibial components (A). Lateral radiograph showing anterior dislocation of the meniscal bearing (B). Photograph of the retrieved meniscal bearing showing severe erosions on the posterior lip, suggesting that wear of the posterior lip during deep flexion was the cause of this dislocation (C). Anteroposterior radiograph after exchange of the meniscal bearing with a thicker bearing (D).

Figure 1:

Postoperative anteroposterior radiograph showing good alignment of the femoral and tibial components (A). Lateral radiograph showing anterior dislocation of the meniscal bearing (B). Photograph of the retrieved meniscal bearing showing severe erosions on the posterior lip, suggesting that wear of the posterior lip during deep flexion was the cause of this dislocation (C). Anteroposterior radiograph after exchange of the meniscal bearing with a thicker bearing (D).

Ten years and 4 months postoperatively, the patient experienced abrupt knee pain after standing up from the floor. Radiographs revealed anterior dislocation of the bearing (Figure 1B). During surgery, the anterior cruciate ligament was intact. Femoral and tibial components were not loose. The retrieved bearing had significant wear on its posterior lip (Figure 1C). The medial collateral ligament appeared intact. However, the gap was slightly wide. Therefore, the retrieved bearing was replaced with a new, thicker (7 mm) bearing (Figure 1D).

Postoperative recovery was satisfactory, with knee range of motion of 0° to 135°. Periodic examinations up to 1 year postoperatively revealed no problem such as bearing redislocation.

Patient 2

A 55-year-old woman (height, 158 cm; weight, 60 kg; body mass index, 24 kg/m2) with medial compartmental osteoarthritis of the right knee underwent an Oxford phase III UKA in December 2004. Using a minimally invasive technique, a 3-mm bearing was implanted. Postoperative radiographs showed well-aligned femoral and tibial components (Figure 2A). Her postoperative recovery was good. The patient was able to return to her job (caregiver), with knee range of motion of 0° to 140°. She was satisfied for more than 11 years.

Postoperative anteroposterior radiograph showing well-aligned femoral and tibial components (A). Lateral radiograph showing the mobile meniscal bearing dislocated anteriorly (B). Photograph of the retrieved meniscal bearing showing macroscopic wear on its superior articular surface in the posterior edge (C). Anteroposterior radiograph after exchange of the worn meniscal bearing with a thicker bearing (D).

Figure 2:

Postoperative anteroposterior radiograph showing well-aligned femoral and tibial components (A). Lateral radiograph showing the mobile meniscal bearing dislocated anteriorly (B). Photograph of the retrieved meniscal bearing showing macroscopic wear on its superior articular surface in the posterior edge (C). Anteroposterior radiograph after exchange of the worn meniscal bearing with a thicker bearing (D).

At 11 years and 6 months postoperatively, the patient presented with painful swelling of the right knee that occurred when she was rising from a low chair. Radiographs showed that the bearing was dislocated anteriorly (Figure 2B). During surgery, the retrieved bearing showed macroscopic wear on its posterior lip (Figure 2C). Femoral and tibial components were well fixed. The anterior cruciate ligament was also in good condition. The worn bearing was replaced with a thicker (6 mm) bearing because of some medial laxity (Figure 2D).

At 1-year follow-up, the patient had no pain. The knee was stable, with range of motion of 0° to 140°.

Discussion

To the best of the authors' knowledge, this is the first report of late bearing dislocation after long-term follow-up of Oxford UKA with a mechanism different from those reported in previous studies. Loss of bump in the posterior lip was found to be the cause.

Lewold et al7 previously documented that most failures of the Oxford UKA occur in the first 2 years postoperatively, with dislocation of the bearing being the most common cause of failure. They reported that bearing dislocation could be attributed to malposition of components and soft tissue imbalance with subsequent maltracking of the bearing. Lisowski et al8 also reported that the most frequent cause of revision in the Oxford UKA is bearing dislocation. Bearing dislocation primarily occurs shortly after implantation, probably due to a technical error.

Kim et al6 suggested that mobile-bearing UKA may be less appropriate for Asians because extreme knee flexion is required by their culture. The reason for the high rate of bearing dislocation in Asians might be inadequate stability of the bearing in high flexion postures such as kneeling, squatting, and sitting cross-legged.

However, despite the risk of bearing dislocation, mobile-bearing UKA remains useful for Asians.9,10 Many studies have reported excellent clinical results after a minimally invasive Oxford UKA.3,5,10,11 Mobile bearings have a lower wear rate compared with fixed bearings. Psychoyios et al12 found a very low rate of polyethylene wear for mobile-bearing UKA. They reported a mean linear penetration wear rate of 0.036 mm/y, with a mean volumetric wear rate of 6 mm3/y. Ashraf et al13 showed that the mean linear penetration wear rate was 0.15 mm/y, with a mean volumetric wear rate of 17.3 mm3/y, for fixed-bearing UKA. This general wear rate could not be applied to the current cases because the wear was limited to the posterior lip of the bearing.

The eccentric wear in the current cases might be due to high knee flexion of the Asian lifestyle. Nagura et al14 reported an increase in net posterior force acting on the tibia during deep flexion. Because a normal tibiofemoral joint decreases its contact area to 55% when the knee is flexed to 90°, significant joint contact force in deep flexion could influence the rate of degenerative changes to the knee over time. As higher flexion becomes available in UKA, higher posterior loads are expected at the bearing, which will result in wear of the posterior lip of the bearing as seen in the current cases.

Another possible cause of polyethylene wear could be soft tissue tightness due to overtensioning of medial soft tissues that causes the femoral component to impinge on the posterior lip of the bearing, similar to poor soft tissue balancing in a cruciate-retaining total knee arthroplasty. This can result in tightness of the posterior cruciate ligament in flexion, leading to posterior polyethylene wear.15 Such posteromedial tightness could cause chronic posterior impingement, eventually leading to bearing dislocation.

The authors discovered common features in late bearing dislocations. First, the problem occurs when the posterior lip of the bearing is worn out. This creates an anterior dislocation of the bearing. Second, satisfactory results can be obtained with simple bearing exchange if there is no abnormality in other components. After late bearing dislocation, conversion to total knee arthroplasty is unnecessary because a simple bearing exchange will suffice. However, 1-year follow-up is not long enough to ensure that there will be no redislocation of the bearing in the future.

Conclusion

A bearing dislocation of the Oxford UKA can occur not only in the early postoperative phase but also in the late postoperative phase, especially for individuals with lifestyles that require high knee flexion. Such late bearing dislocation has a mechanism different from those previously reported. Loss of bump in the posterior lip was found to be the cause of late bearing dislocation. Conversion to total knee arthroplasty immediately after the dislocation is not imperative because a bearing exchange could yield a favorable outcome. From a long-term perspective, appropriate education and lifestyle change are necessary after bearing dislocation because high knee flexion could have been the cause.

References

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Authors

The authors are from the Department of Orthopaedic Surgery, Dong-Eui Medical Center, Busan, Korea.

The authors have no relevant financial relationships to disclose.

Correspondence should be addressed to: Moo-Ho Song, MD, PhD, Department of Orthopaedic Surgery, Dong-Eui Medical Center, Yangjeong-ro, Busanjin-gu, Busan 47227, Korea ( mhsong21@hanmail.net).

Received: December 02, 2017
Accepted: April 19, 2018
Posted Online: October 16, 2018

10.3928/01477447-20181010-09

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