CASE 4: Revision of a metal-on-metal hip with leg shortening using a cup augment
The Story
“Steven presented to my clinic at the age of 60 with pain in his right hip and a leg length discrepancy (short on the right side). He had suffered with osteoarthritis and received a right metal-on-metal Birmingham hip resurfacing to treat his OA three years prior to our meeting.
As with any painful hip replacement, we had to consider the common causes of loosening, infection or an inflammatory process secondary to his metal-on-metal bearing.”
The Investigation
Clinical examination of Steven revealed a negative Trendelenburg test, a stiff right hip and trochanteric tenderness on the right hand side.
His Oxford Hip Score on the right side was 29/48.
We investigated Steven with blood inflammatory markers which were raised (CRP = 20mg/L) and performed a hip aspiration to help rule out infection. After 14-days of extended culture, this was negative. As part of the hip aspiration procedure, we injected some local anaesthetic into his right hip which relieved his symptoms for 24-hours.
Steven’s blood cobalt and chromium levels were below 7ppb (2.9ppb and 2.7ppb respectively).
The Evidence
Anteroposterior plain radiograph of the pelvis showing a well fixed Birmingham right hip resurfacing. The right component is slightly large and the cup inclination is low at 22 degrees. A CT scan revealed that the cup was anteverted at 40 degrees.
The Diagnosis
Steven was diagnosed with a painful right Birmingham hip resurfacing secondary to metal ion synovitis. Differential diagnosis includes mechanical symptoms due to the cup position: high centre of rotation and low cup inclination (which can cause impingement on femoral neck and psoas tendon).
The Plan
We planned to use an augment to try and lower the centre of rotation of the acetabular cup and achieve good fixation despite the presence of a superior acetabular wall defect.
Augment allows the surgeon to reconstruct a socket that has poor underlying bone stock. It is secured to the bone by screws, and acts as a buttress for the acetabular cup to fit in against.
The Operation
We used a posterior approach to access Steven’s right hip joint and protected his sciatic nerve. We then proceeded to make the femoral neck cut and explant the cup. A superior acetabular wall defect was noted. Bone graft was used with a superior augment, a 56mm cup and screws to achieve good fixation. The femoral side was then replaced and the joint was checked for stability, range of movement and leg length. The surgical site was washed and closed.
The Outcome
Anteroposterior plain radiograph of the pelvis shows a Corail stem and Pinnacle Gription acetabular socket with superior augment. The centre of rotation of the and cup angles have been optimised. A ceramic-on-polyethylene bearing surface was used.
Anteroposterior plain radiograph taken at one year after Steven’s operation showing that the augment and the cup have not migrated or rotated.
Long leg plain radiograph showing that the patients leg length discrepancy has been corrected by revision surgery.
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One year down the line and Stephen had returned back to his normal daily activities and had returned to doing more exercise. He was very happy with the surgical correction of his leg length discrepancy.
The Verdict
“Augments can be used to restore the centre of rotation of the cup and maintain appropriate cup size to avoid the loss of anterior and posterior columns as seen with jumbo cups. This helps to preserve bone stock surrounding the acetabulum and reduces the surgical recovery time and morbidity.
This case highlights another important point about the positioning of cups. A low cup inclination angle with a high anteversion angle is an issue because it can cause lateral impingement at low levels of abduction. This can affect walking and other normal daily activities. It also causes the shedding of metal ions which can irritate the soft tissue surrounding the hip. It is likely that the position of Stephen’s acetabular cup sped up the process of metal ion shedding, leading to irritation early in the implant’s life.”
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Technical / surgical difficulties in achieving optimal implant size, position and orientation were common with metal on metal hip resurfacing for several reasons.
The preservation of the femoral head necessitated a larger soft tissue dissection (often a longer skin incision, 360 degree capsulotomy, and tendonotomy of gluteus maximus) to gain adequate surgical exposure of the acetabulum. This required surgical experience and is more challenging in large and muscular males (often the most suited for hip resurfacing, and in fact hip resurfacing remains banned in females and small males in the UK).
The most feared complication in the early years of re-introduction of hip resurfacing (re-introduced in 1997 in the UK) was femoral neck fracture and so it is common is see over-sized femoral components to minimise femoral neck notching and the risk of femoral neck fracture. Oversizing of the cup also results because the femoral head size determines the acetabular cup size.
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The reasons for revision of metal on metal hip resurfacing involve surgical, implant and patient factors. Another case of revision of metal on metal hip resurfacings can be found here: Case 10
