Hip Dysplasia

The aim is to restore an anatomical centre of rotation whilst maintaining sufficient bony coverage for a stable acetabulum, and not putting excess tension on the sciatic nerve.

Dysplastic hips share a common pattern of anatomic abnormalities. The deficiency is mainly on the acetabular side. Occasionally the whole pelvis is underdeveloped.

The acetabulum (pelvis) is usually shallow; deficient anteriorly and superiorly, with a decreased antero-posterior diameter and antiverted.

The Femoral side (thigh bone) shows persistent femoral antiversion, valgus neck, hypoplastic, narrow canal, small and posteriorly placed greater trochanter.

The combination of these abnormalities leads to a significant reduction in the contact area between the femoral head and acetabulum. As a result, high forces are transmitted through a limited surface area and the increased point loading, which results in articular degenerative changes and eventually osteoarthritis.

These are usually associated with proximal migration of the femur. These include hypoplastic, short muscles acting on the hip joint, thickened joint capsule femoral nerve displacement proximally and laterally and sciatic nerve shortening.

Patients with dysplasia typically present with groin pain that increases with exercise. Other features such as catching, locking and giving way are symptoms that may indicate associated labral tear or articular cartilage pathology.

Diagnosis is made by plain radiograph. Different classification systems are designed to assess the severity and help with choosing the best method of treatment.

Hartofilikadis classification. This classification is reasonably simple:

Type 1: The hip is Dysplastic, but the femoral head remains in the true acetabulum with the femoral head centre and the acetabular edge angle is less than 20 degrees.

Type 2: is described as low hip dislocation where the head articulates in a false acetabulum whose inferior lip overlaps the true acetabulum.

Type 3: high dislocation where false acetabulum has no contact with the true acetabulum.

Crowe’s classification: Measures proximal migration as a proportion of the height of the femoral head compared to the distance from inter tear-drop line.

Type 1 – Proximal migration < 50% height of head (from the inter tear-drop line).
Type 2 – 50-75% proximal migration.
Type 3 – 75-100% proximal migration.
Type 4 - > 100%.

Hip arthritis is the end result of hip dysplasia for which total hip arthroplasty is the procedure of choice for most patients with symptomatic hip arthritis.

The anatomic abnormalities associated with the dysplastic hip increase the complexity of hip arthroplasty. In principle it is desirable to restore normal anatomy and use an uncemented implant. In order to restore anatomy the metal shell needs to be inserted near the normal anatomical position of the acetabular in order to restore hip centre of rotation.  The uncemented acetabular components allow biologic fixation with potentially improved results compared with cemented cups, especially in young patients.

Femoral shortening is required in some cases due to soft tissue shortening, which includes the sciatic nerve where it is at most risk if the leg is lengthened by more than 4 centimeters’. Shortening can be achieved by subtrochanteric osteotomy. The results of total hip arthroplasty demonstrates a high rate of pain relief and functional improvement.

Acetabular reconstruction: Placement of hip centre

  • High
  • Normal
  • In-between - Centralised

Femoral reconstruction: Has 2 considerations

  • Leg length. Can restore limb length via either restoring hip centre or lengthening the femoral component.
  • Abnormal femoral anatomy that needs a small implant using a special surgical technique.

Acetabular reconstruction of the cup placement will be dictated by 2 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:

 

High hip centre

Anatomic centre

Pros

  • Technically easier
  • Avoids need for graft
  • Better biomechanically
  • Less loosening
  • Restoration of leg length
  • Restoration of bone stock

Cons

  • Abnormal biomechanics
  • Bone stock is not restored, making revisions very difficult
  • Higher rates of component loosening
  • Use of smaller cups needed
  • Potential high rate of dislocation due to impingement against acetabulum
  • Often needs bone grafting (shelf) to obtain superior coverage
  • Technically more difficult

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 since it has shown very poor results.
  2. Cotyloplasty (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 would provide more superior coverage to the implant.

The technique used is individualised in each patient and depends upon surgeon 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, though small, allows for better superior coverage than in previous group.

Leg length:

Femoral reconstruction. There are 2 considerations:

  1. The leg length itself.
  2. The abnormal femoral anatomy.

Leg length:

Patients generally want their leg length restored. Techniques used to restore leg length include:

  1. Restoration of hip centre.
  2. Lengthening of the femoral component.

A major limiting factor is the sciatic nerve. May safely lengthen up to 4cm or 6% of limb length (whichever is lesser). Strategies to protect the nerve include:

  1. Trial reduction with knee flexed, and palpate the nerve assessing its tension while gently extending the knee.
  2. Somatosensory Evoked Potentials (SSEP’s) or motor electrophysiological tests.
  3. Wake-up test if nerve is under tension.

Femoral shortening: May be needed if soft tissue will not allow reduction, or if lengthening will increase tension on sciatic nerve. Technical options include:

  1. 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 with 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.
  2. 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 lower potential of healing.

Posterior approach is used, after preparing the femoral stem the osteoto my is done at the subtrochanteric region then the trial stem is inserted and the hip is reduced so the amount of definitive resection could be assessed exactly, then resection of the segment is performed, this is followed by insertion of the definite prosthesis. The resected segment is then longitudinally split and wrapped around the osteotomy site, then reinforced by 2 cables.

Altered femoral anatomy:

The canal can be very small, wider antero-posterior than medial-lateral and excess antiversion. Thus to overcome this stems with small diameters (5-10mm) should be available, using a stem 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 that all components are available and performing accurate assessment of leg length.

Type I hips can be reached through a standard posterior approach, for more exposure consider a trochanteric slide, Chevron or flat osteotomy at base of the greater trochanter, retract to expose more of ilium, reattach via wires. To assess how deep you can ream drill through the floor and measure how thick it is, ream to within 0.5-1.0cm and insert trial if < 70% is covered use the femoral head to create a shelf.

Acetabular component has better results with uncemented cups, more native bone coverage and positioning the component as near as to the anatomical hip centre.

Femoral component shows that cemented femurs perform better than cemented cups. Uncemented femurs show promise in younger patients since they are more biological implants. Subtrochanteric osteotomy has 80% satisfactory results which are better with uncemented implants.