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In a dysplastic hip there two approaches to restoring function. The approach taken and acetabular reconstruction of the cup placement is influenced by two factors:
1. Bone stock. High hip centres have poor bone stock (particularly if they need to be revised). Anatomical centres in severe dysplasia will require superior augmentation such as shelf auto-graft in order to obtain sufficient superior coverage.
2. Limb length discrepancy.
There are pros and cons to each approach, and each has its advocates and its critics:
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High hip centre |
Anatomic centre |
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Cons |
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Technical options to include superior coverage include:
1. Shelf augmentation: Where the femoral head could be used as autograft. Note that cement cannot be used for superior coverage as it has shown very poor results
2. Cotyloplasty: This is a technique that involves making a perforation of the medial wall of a shallow acetabulum and then inserting an acetabular cup with the medial aspect of its dome beyond the Kohler line. This leads to medialisation of the cup by controlled medial wall fracture and bone grafting. This provides more superior coverage to the implant
The technique used is individual to each patient and their individual hip dysplasia categorisation. The approach and technique used also depends upon the surgeons preference and training. In general:
Leg length:
With a femoral reconstruction there are two considerations:
1. The leg length itself
2. The abnormal femoral anatomy
Patients generally want their leg length restored. Techniques used to restore leg length include:
A major limiting factor is the sciatic nerve. To protect the nerve you can safely lengthen up to 4cm or 6% of limb length (whichever is lesser).
Strategies to protect the sciatic nerve include:
Femoral shortening:
Femoral shortening is needed if soft tissue will not allow reduction or if lengthening will increase tension on sciatic nerve. Technical options include:
A posterior approach is used after preparing the femoral stem and the osteotomy is done at the subtrochanteric region. The trial stem is then inserted and the hip is reduced so the amount of definitive resection can be assessed exactly. This is followed by resection of the segment and then insertion of the definite prosthesis. The resected segment is then longitudinally split and wrapped around the osteotomy site and reinforced by two cables.
Altered femoral anatomy:
The canal can be very small and wider antero-posterior than medial-lateral with excess antiversion. Thus to overcome this stems with small diameters (5-10mm) are available to use on which the antiversion can be set independent to the anatomical antiversion such as a modular stem or cemented stem with very small metaphyseal flare (DDH stems).
Need for small components:
Pre-operative planning should include special technical considerations such as ensuring all components are available and performing accurate assessment of leg length.