Fracture of femoral component in an Oxford^® unicompartmental knee replacement with displaced posterior fragment: a case report

Abstract

Femur component fracture is very rare with Oxford^® unicompartmental knee replacement. We report a case from the UK with displacement and rotation of the broken femoral component posterior to the peg. Potential predisposing factors include polywear with metal on metal articulation. The patient had excellent results following revision arthroplasty.

Introduction

Unicompartmental knee replacement is a common primary knee arthroplasty procedure in the UK. 96,700 unicondylar knee replacements were performed in the UK until the most recent National Joint Registry (NJR) report in 2019. Oxford^® unicondylar knee procedures still lead the list, amounting to 60% of all partial knee replacements. The Oxford^® unicondylar knee replacement is a very successful procedure. Reported complications include dislocation of the mobile bearing, loosening of the prosthesis, periprosthetic fracture, polyethylene wear, progression of arthritis in the contralateral compartment, medial collateral ligament injury, impingement, and infection. The most common complication following a unicondylar knee arthroplasty (UKA) was a mobile bearing dislocation in the mobile bearing knees and a loosening of the prosthesis in the fixed bearing knees. Here, however, we are reporting on a very uncommon complication following Oxford^® unicondylar knee replacement, where there was a fracture of the femoral component. The fractured posterior metal femoral fragment rotated by 180° and was displaced into the popliteal gutter.

Case history

A 65-year-old man attended the emergency department with a painful knee replacement episode. He had an Oxford^® unicondylar knee replacement 13 years ago. Following the procedure, he led a very active lifestyle. In recent months, however, he noticed intermittent pain, swelling and stiffness in his knee. He also had to swing his knee more than usual to mobilize. Additionally, the man had several episodes of knee locking which he had to manipulate to unlock.

He attended the emergency department with acute painful locking of his knee. The man was getting out the car and his toe got caught inside the vehicle. His knee became twisted and got locked in flexion. The individual could not fully unlock his knee.

Clinically, his knee was swollen and deformed. The patient had flexion deformity of 30° as well as varus deformity. His knee’s range of motion was limited. Systemically, the individual was well with no clinical and laboratory features suggestive of infection.

An x-ray of his knee [Fig 1] showed femoral component fracture. The posterior fragment was rotated 180° and it was dislocated into the popliteal area. The mobile bearing insert had been displaced posteriorly along with a fractured femoral component, ultimately resulting in metal on metal articulation. The tibial component appeared well fixed and secure.

Figure 1.

Anteroposterior and lateral view x-ray

We aspirated his knee to rule out infection. Following this, the man had a single stage revision arthroplasty. During the revision, the displaced posterior fragment was removed with some difficulty. The anterior part could be removed easily, while bone loss was very minimal on the femoral side [Fig 2]. The tibial component was removed with slight bone loss. There was, however, evidence of metallosis and polyethylene wear in the knee. A posteriorly-stabilised Genesis^TM II femur component with a stemmed tibia was used. The patella then resurfaced. Tissue samples were taken and sent for microbiology.

Figure 2.

Intraoperative pictures show loose and broken femoral component

The patient had an uneventful recovery and his microbiology cultures were negative. At the man’s 6-month follow-up, his range of motion was 0-110° and he had a stable, well-functioning knee replacement [Fig 3].

Figure 3.

Post-operative x-rays

Discussion

The most common complication after UKA is dislocation of the mobile bearing. There had only been few reported femoral component fractures in the literature. Our case is unique insofar as there were no predisposing factors such as malalignment of the components, high BMI or painful knee due to aseptic loosening: the patient had a well-functioning UKA. In our case, the femoral component fractured posterior to the single peg of the Oxford^® femoral component and the worn out bearing had dislocated posteriorly. The femoral component posterior to the peg had rotated 180° and had displaced back into the popliteal area. We believe that the reason for failure was polywear and fatigue failure posterior to the peg due to metal on metal articulation. Thin posterior cement mantle also contributed. Many previously reported cases of femoral component fractures were older designs, which have since gone out of use.^2–4 Some of the reported fractures were anterior to the peg of the femoral component.⁵ In 1986, Sandborn et al reported a fracture of the femoral component anterior to the peg in a Richards modular unicompartmental knee replacement.⁶ The reason for failure was fatigue fracture in the sub-optimally positioned steel femoral component. In 2012, van der Veen and van Raay reported fracture in an Oxford^® unicompartmental knee femoral component. In their report, high BMI as well as tibial and femoral malalignment had contributed to the loosening and fracture of the femoral component. In our case, the patient had led a very active lifestyle, his BMI was not high and he did not present any problems for 13 years.

Conclusions

Femoral component fracture in an Oxford^® unicompartmental knee arthroplasty is a very uncommon complication. There is paucity in the literature. Other reported cases belong to the older designs which are not in use anymore. It is also noticeable that similar problems were not reported in Oxford^® unicompartmental knees with double peg femoral components. In view of this, a single peg femoral component may be more prone to fatigue fracture compared to double peg components, where the force is more evenly distributed. With improvements in design, materials and with updated techniques, we hope that similar problems will not occur in the future.