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Hip Dysplasia - Hip Centre
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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Pros |
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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:
- Hartofilakidis I / Crowe I – generally easy to return to anatomical centre
- Hartofilakidis II / Crowe II and III – most difficult to return to anatomical centre because when the false acetabulum is in continuity with the true acetabulum the resulting superior bone stock deficiency is greatest
- Hart. III / Crowe IV – Somewhat easier to return to anatomical centre (providing lengthening allows it). The true acetabulum is not eroded superiorly and although small allows for better superior coverage than in previous group
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:
- Restoration of hip centre
- Lengthening of the femoral component
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:
- Trial reduction with knee flexed and palpate the nerve assessing its tension while gently extending the knee
- Somatosensory Evoked Potentials (SSEP’s) or motor electrophysiological tests
- Wake-up test if nerve is under tension
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:
- Sequential proximal resection: This technique involves greater trochanter osteotomy, sequential proximal resection and insertion of a cemented stem. It is technically easier but there is a risk of greater trochanteric non-union resulting in the proximal femur becoming just a straight tube (with no metaphyseal flare) and thus will only accept cemented prostheses. Very distal resection will remove the lesser trochanter with the psoas muscle insertion which leads to decreased flexion strength.
- Subtrochanteric osteotomy: This technique has the benefit of maintaining the important metaphyseal anatomy and bone stock, as well as allowing for correction of antiversion abnormality by rotating the proximal segment. This is technically challenging and has a higher rate of non-union due to the fact that osteotomy is made at diaphysial bone, which has a lower potential of healing.
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:
- Cups with an outer diameter as small as 36mm are needed, as well as thickest possible poly, minimum 8mm. This may lead to the use of a very small head
- Femoral stems require small diameters 5-10mm with variable metaphyseal sleeves or sizes such as modular non-cemented or DDH cemented stems
Pre-operative planning should include special technical considerations such as ensuring all components are available and performing accurate assessment of leg length.
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