Rare Asymptomatic Posterior Dislocation of Mobile Bearing Insert in Unicondylar Knee Arthroplasty: A Case Report

Abstract

Dislocation of polyethylene insert is a common complication of mobile bearing uniconylar knee arthroplasty (UKA). Dislocation, when occurs is common in anterior, lateral or medial direction. The authors report a case of posterior dislocation in which patient remained asymptomatic and the dislocation was detected in a routine follow-up radiograph done at 18 months after surgery. UKA was subsequently converted into total knee arthroplasty and patient had a stable knee at two years follow-up with no signs of loosening or instability. Posterior meniscal bearing dislocation may remain asymptomatic and continued examination with high index of suspicion is needed to disclose such complications in the absence of symptoms.

Introduction

Bearing dislocation is one of the earlier occurring complications of mobile-bearing (MB) Oxford uniconylar knee arthroplasty (UKA) with reported incidence rates between 0 and 5.3% [1]. A fully congruent MB polyethylene (poly) insert, used in Oxford UKA, closely approximates normal knee kinematics. Theoretically, it has a lesser risk of component loosening and low wear potential but carries risk of dislocation. Dislocation, when it occurs, is usually anterior and rarely occurs in other direction [2–4]

We report a case of rare posterior dislocation of the MB insert as an incidental finding during a regular 18 months follow-up of a case of medial Oxford UKA. Index surgery was done in our institute which is a tertiary care hospital, by a high volume joint replacement surgeon (RM). Consent of the patient was taken before sending this case report for publication.

A 57 years old, obese female (BMI 29.3 kg/mt²) presented in out-patient department (OPD) in October, 2017 with complaints of bilateral knee pain with pain in the left knee confined only over the medial portion of the knee. A detailed clinical and radiological evaluation suggested tricompartmental osteoarthritis of right knee and medial compartment arthritis of left knee (Fig. 1a, b). After medical evaluation and anesthetic clearance patient underwent total knee arthroplasty (THA) for right side and UKA for left side. Intra-operatively during UKA trial, size 7 poly gave a stable articulation and knee was well balanced in coronal plane with no signs of dislocation of insert. Extra small femoral and AA size tibial component was used. Post-operative period was uneventful and patient was discharged on third post-operative day (Fig. 2a, b).

Fig. 1.

a Preoperative (Index surgery) AP knee radiograph showing primarily medial compartment arthritis with typical ‘bone on bone contact’. The lateral compartment has a nearly ‘full thickness’ joint space visible on the AP view. b Lateral view showing fairly preserved the patellofemoral joint

Fig. 2.

a Post-operative knee AP radiograph showing insert in correct position. b Knee lateral view radiograph. c Post-operative radiograph of index surgery showing increased tibio-femoral angle with 9.83° valgus aligmnent due to a large (size7) poly insert

Patient was regular in OPD follow-up with no signs of infection or instability. Patient achieved full range of motion (ROM) at six weeks follow-up and was able to regain normal lifestyle at three months post-surgery. Patient visited our OPD at 18 months for routine follow-up with no clinically relevant symptoms. Patient had full painfree ROM and fairly normal gait. A radiograph of both knees was still ordered without any significant symptoms. X-ray films showed, a well maintained alignment of femur and tibia components of TKA on right side, while on left side, the MB insert was found to be vertically snipped in the posterior compartment (as suggested by the metal marker inside the poly insert) (Fig. 3a, b). Even on taking detailed history, patient could not recall any specific traumatic episode leading to pain or swelling. Careful observation showed slight varus appearance of the left lower limb. On further clinical examination, there was varus laxity with metal on metal sound on varus stress.

Fig. 3.

a Dislocated poly insert in knee AP view radiograph (arrow indicating metallic marker). b Posteriorly dislocated poly inset (indicated by metallic marker inside the poly insert) (arrow)

The clinicoradiological diagnosis of posteriorly dislocated poly insert was made. The patient was explained the need of revision surgery in view of dislocated insert, which was difficult for her to comprehend as she was completely asymptomatic after surgery. After thorough counseling and considering various revision options for a failed UKA, decision for conversion to TKA was made in this case. Intra-operatively, intense metal reaction with blackish synovial hypertrophy was seen. The femoral and tibial components were well fixed with bearing being displaced posterolaterally and anterior to the posterior capsule. There were signs of wear on the both components due to metal on metal contact during weight bearing (Fig. 4a, b). Components were removed cautiously preserving as much bone as possible. A posterior stabilizing knee TKA (Stryker© Scorpio) was done without the need of any augmentation (Fig. 5a, b). Post-operatively, patient was put on routine TKA rehabilitation protocol. At two years follow-up, patient was doing well with ROM from 0° to 110°. The knee was stable on examination with no evidence loosening, infection or instability.

Fig. 4.

a Surface wear in the form of shallow erosion seen on extracted tibial component (arrow). b Extracted femoral component showing surface wear (arrow)

Fig. 5.

a Post-operative total knee replacement done knee AP view radiograph. b Knee lateral view radiograph

Discussion

Primary bearing dislocation commonly occurs due to component malposition, impingement with remnant osteophytes or meniscus component, distraction due to inadequately balanced flexion–extension gap, laxity due to ligament release or instability due to medial collateral ligament (MCL) injury [1]. Dislocated bearing inserts are usually found on the anterior, medial, or lateral side of the intracapsular area. Even in in vitro experimental studies, done on Oxford rig much greater resistance was seen for posterior bearing dislocation [3]. Since the anterior rim of the insert is higher than the posterior rim, posterior dislocation is unlikely unless spinning of the bearing in a distracted compartment occurs.

In our case, retrospective evaluation of postoperative radiograph of index surgery revealed slight overcorrection of varus with a large sized poly which pushed the knee into valgus with tibiofemoral angle of 9.83° (Fig. 2c). Post-operative tibio-femoral angle is an important determinant for the outcome of surgery and stability of the implant. Higher failure rates have been reported in undercorrected knee with varus greater than 3° or the knee with overcorrection and postoperative valgus of greater than 7° [5]. Overcorrections into valgus could lead to higher stresses in the lateral compartment and stress shielding under the implant [6]. We suspect that patient might have dislocated the poly during kneeling or sitting cross-legged during household activities. Higher propensity of bearing dislocation have been reported in Asian population as compared to Western population owning to high knee flexion behavior in their social and religious practices in the former [7].

Even on close probing, patient could not recall any specific traumatic event, and surprisingly, remained asymptomatic. Bearing dislocation with faint symptoms or no pain has been scarcely reported in the literature [8, 9], with patient remaining symptom-free after dislocation for as long as 6 years in a report [8]. There has been controversy in the literature regarding the indications of UKA. Traditionally, high BMI has been considered a contraindication for UKA. However, several recent studies have refuted this notion and reported no statistically significant difference in the clinical or functional outcome, failure rate or survival of UKA when compared with non-obese patients [10–12]. Similar observations have been made in studies investigating results of UKA in presence of low grade patellofemoral arthritis [10, 13, 14]. Thus in this case, an obese patient with mild patellofemoral involvement but having significant anteromedial knee arthritis (Fig. 1a, b), UKA was a viable option at the index surgery.

A failed UKA to TKA conversion is a technically demanding procedure. In this case, poly exchange with an insert bigger than 7 mm would have further increased valgus malalignment. Reusing the worn-out components with shallow surface erosion was also a concern (Fig. 4a, b). Moreover, high rate of redislocation after poly-exchange have been reported in literature [1], with upto 50% in some studies [15]. Thus, poly exchange or revision UKA was not preferred. In the presence of knee laxity, obesity and a demand for a single surgery remedial procedure, TKA was done.

Conclusion

Posterior dislocation of meniscal bearing can occur in obese patients in an overcorrected UKA, especially when patient performs high knee flexion activities routinely. Such a complication may go unnoticed due to lack of any relevant symptoms, therefore a high index of clinical suspicion is required in such cases. Optimum component placement, appropriate soft tissue balance, integrity of ligaments and post-operative varus–valgus alignment are important determinants of success of surgery.

Compliance with Ethical Standards

Conflict of interest

The authors declare that they have no conflict of interest.

Ethical standard statement

This article does not contain any studies with human or animal subjects performed by the any of the authors.

Informed consent

For this type of study informed consent is not required.