Arthroscopically-assisted retrieval of a fractured polyethylene Oxford knee meniscal bearing: a case report

Introduction

Unicompartmental knee arthroplasty (UKA) is an established treatment in the management of end-stage medial compartment osteoarthritis, providing good pain relief and restoring function. ¹ The Oxford knee is the most widely-used UKA, with long-term survival comparable to that achieved with total knee replacement. ¹ The Oxford knee has a mobile bearing with the advantages of reduced wear and early failure. However these advantages make the device vulnerable to dislocation, which although rare, has an incidence of 0.5% in medial UKA and up to 10% in lateral UKA. ² There are few reports of breakage of the mobile bearing following Oxford knee medial unicompartmental replacement.

The aim of this paper was to present our experience with a minimally-invasive treatment option for retrieving a posteriorly-migrated fragment of a mobile tibial bearing in a case with this rare complication.

Case report

A man in his mid-50s underwent Oxford medial UKA in September 2014 following previous anterior cruciate ligament reconstruction. A medium-sized femoral component, size E tibial tray and a medium size 3 meniscal bearing provided stable articulation. Postoperatively, the knee remained pain-free for 2 years following the original operation until the patient sustained a twisting injury to his knee. Following this injury, he developed pain and swelling in the left knee and was unable to bear weight. Objectively, preoperative range of motion (ROM) was decreased compared with normal, with passive ROM ranging from 20° to 90° and active ROM from 20° to 60°. Radiographs demonstrated breakage of the insert, with half dislocated to the suprapatellar recess and half dislocated posteriorly to the femoral condyle (Figure 1).

OPEN IN VIEWER

Figure 1.

Radiograph showing the breakage and dislocation of the two parts of the bearing (red arrow and red circle).

Patient consent for treatment was obtained. Surgery was performed under regional anesthesia using femoral and sciatic nerve blocks (bi-block) and with the use of a tourniquet. The foot of the operating table was lowered so that the affected knee could hang free at the edge of the table in 90° of flexion. Routine arthroscopic examination of the knee joint was performed through the standard anterolateral and anteromedial ports. This allowed the visualization of the half of the insert that had migrated to the suprapatellar recess, and this fragment was extracted through a mini-anteromedial incision on the mid-portion of the previous surgical scar. To approach the posteromedial knee compartment, a posteromedial port was established under direct arthroscopic visualization with the help of a spinal needle. Using a straight Kocher forceps through the posteromedial port, the fragment of the insert was identified, dislocated medially. The fragment was extracted through the same mini-anteromedial incision (Figure 2). Both femoral and tibial components were well fixed with no macroscopic evidence of burnishing, abrasions or scratching over the metal surfaces. The cruciate and collateral ligaments were intact. A new mobile bearing of the same size was inserted through the mini-incision (Figure 3). The wound was closed, and a sterile dressing was applied. Active and passive ROM of the knee and isometric muscle exercises were started the day after the operation, and the patient was mobile and fully weight-bearing, without difficulties. Four years after the surgery, the patient’s Knee Osteoarthritis Outcome Score was 79. He was relatively pain-free and continued to enjoy normal function of the left knee.

OPEN IN VIEWER

Figure 2.

Photograph of the broken insert after extraction.

OPEN IN VIEWER

Figure 3.

Postoperative radiographs showing correct placement of the new bearing.

The reporting of this study conforms to the CARE guidelines. ³ All patient details have been de-identified.

Discussion

Common causes for bearing failure following UKA comprise malpositioning of the component, inadequate size of the insert, residual ligamental laxity, knee deformities or wear. ⁴ Traumatic injuries can lead to bearing dislocation or in rare cases, to breakage of the component. In a report by Lim et al., a worn Oxford III meniscal bearing fractured 7 years after primary surgery as a result of significant wear and was replaced with a thicker bearing. ⁵ Recently, Vajapey et al. reported a case of meniscal bearing rupture that was treated with total knee arthroplasty, given the concomitant development of arthritis in the other compartments. ⁶ Dislocation can be treated by manipulation and relocation of the meniscus, which sometimes occurs spontaneously. ⁷ However, arthrotomy through the old anteromedial incision is almost always required to remove the bearing and to determine and rectify the cause of its displacement. The bearing can usually be retrieved through the anterior incision even if it has displaced to the back of the joint. However, sometimes, the bearing cannot be found through the anteromedial approach, if the bearing has migrated well below the joint line. Tibrewal et al ⁸ reported two cases in which a posteriorly migrated bearing could not be found through the anteromedial approach and was left in situ without further complaints from the patients. A diagnosis of bearing dislocation can be made on the basis of the clinical history and physical examination, although in some cases, symptoms may be minimal. Plain radiographs are diagnostic. Direct multiplanar computed tomography (CT) images are useful as these provide excellent detail regarding the surrounding structures.

This case report demonstrates that in the event of breakage of the bearing with posterior dislocation of half of the bearing, which may be difficult to retrieve through the original mini-arthrotomy, an arthroscopically-assisted approach is safe to help remove the bearing and allow its replacement without creating a separate posterior incision. According to previous studies, insert breakages are most likely to occur with bearings measuring ≤4 mm in size,^5,6 as in our experience. Lim et al., replaced a 4-mm broken meniscal bearing with a 6-mm bearing. ⁵ In our case, no evidence of malpositioning of the component or ligamental laxities were identified. Therefore, the broken insert was replaced with one of the same size to avoid over-correction of varus deformity and the potential for increased load to the lateral compartment.

Supplemental Material