Protrusio acetabuli is a defect of the central acetabulum resulting from migration of the femoral head medial to the Ilio-ischial (Kohler’s) line. There are a number of causes, the common ones being secondary to inflammatory cause (inflammatory destruction). Other causes include metabolic, post traumatic, osteogenesis imperfecta, Paget’s disease. The common factor is a metabolic or mechanical alteration of the peri-acetabular bone.
Central migration occurs along the resultant joint-reaction force vector. Total hip arthroplasty has been the traditional surgical treatment and requires reconstruction of the contained central bony defect with bone graft. Lateralization of the cup is necessary to a successful outcome. Careful pre-operative planning is required to assess limb length, implant choice, size and position. The outcome of THR with bone grafting for protrusion has been favorable with success rates comparable to primary THR. However since many patients are young or middle aged, a revision procedure may be required. Therefore the goal of the primary arthroplasty should be to ensure long survivorship of the implant. Traditional survival rates for cementless THR in patients less than 55 years ranges between 90 % at 10 years to 60 % at 15 years.
Therefore it is vital to choose appropriate implants to increase survival rates. The surgeon can influence this by choosing bearing couples and implant design. In this case an alternative to a total hip arthroplasty has been performed for a severe grade of Protrusio acetabuli. The implant chosen was the short stem Proxima hip (Depuy).
This consists of a bone conserving stem less modular femoral component. It gains purchase in the cancellous proximal bone of the femur (Hence the name Proxima). The bearing on the femoral side was a 36 mm dia Delta ceramic head. On the acetabular side a cementless Pinnacle cup with a metal shell and metal liner was chosen.
A 25 year young Indian male presented with short limbed gait and restriction of movements. This resulted from a long standing central fracture dislocation of the acetabulum treated conservatively. Llimb length discrepancy (LLD) was one inch. Movements were severely restricted but pain was not significant.
He was unable to flex his hips beyond 70 degrees. Figure 1 shows the pre-operative x-ray indicating a grade III protrusio and shortening. Hip resurfacing was contra indicated owing to the non-anatomical position of the acetabulum, limb length discrepancy and necessity for bone grafting
Principles of hip reconstruction for Protrusio acetabuli
Exposure should be liberal to deliver the femoral head. The Sciatic nerve should be identified and protected as it lies superficial. The head is difficult to dislocate and may have to be osteotomized in situ and removed piece-meal. Acetabular reaming should be restricted to the periphery to enlarge the rim and obtain a peripheral fit.
Cartilage from the rest of the acetabulum is removed with a curette. In the more severe grades, bone grafting is mandatory. An anti protrusio cage can be used to reconstruct a severe medial wall deficiency. l Screw fixation augments the peripheral fit. Limb length discrepancy can be addressed with a longer neck length bone grafting to the central defect and lateralization of the acetabulum.
The cup was lateralized by morsellized bone graft obtained from the femoral head. Peripheral fit and additional screw fixation achieved a snug interference fit. The cup was reamed to 54 mm and a 54 mm Pinnacle cup was used. (Depuy) Stable peripheral fit was obtained with the 54 mm cup. Additional fixation was secured with two 6. 5mm cancellous screws. A 54 mm metal liner was used.
On the femoral side femoral neck resection was performed higher than for a conventional THA, just below the head level. Broaching was done in a round the corner technique. A size 2 Proxima stem was chosen as it conferred three dimensional stability. A medium neck length was chosen. During trialing, lack of telescopy, good reduction and coverage was obtained. Limb length was also checked and found to be acceptable with a medium neck length. Real components size 2 Proxima stem (Depuy), 36 mm diameter Delta Ceramic head (Biolox) were chosen as the definitive implants.
The Proxima hip is an anatomic implant with a pronounced lateral flare and stem less stem. It gains purchase in the proximal cancellous bone of the femur. It sacrifices very little bone from the head of the femur. Most of the neck portion is left intact. It has a HA coating. It is one amongst short stem hip replacements. This short stem is neck preserving. The rationale for short stems in cementless (THA) is proximal load transfer and absence of distal fixation. This results in preserved femoral bone stock and avoidance of thigh pain. The surgical technique is different from a standard total hip replacement. Round the corner broaching is done.
The advantages of a Proxima hip over a total hip are
In carefully selected young patients such as this one, use of the Proxima short stem appears as a simple better and effective option to THA.