Most slips cause acetabular damage from impingement. Neck osteoplasty can eliminate impingement in milder deformities. Osteotomy is required to correct severe deformities. Distal osteotomy should be combined with osteoplasty to decrease the amount of correction required and the secondary deformity created. The Dunn and neck osteotomy correct the deformity perfectly when performed with proper technique.
Unstable Slips
Unstable slips should be reduced to a stable position urgently and stabilized with two fixation points without stretching or kinking of the retinaculum. Ischemia of the head should be diagnosed early and collapse prevented. Evaluation of blood flow allows for correction of reversible ischemia and diagnosis of irreversible ischemia. Pulsatile bleeding from a drill hole in the head is adequate in most cases.
The surgical dislocation approach allows direct visualization and optimization of the stretch on the lateral retinaculum while removing all callus and reducing the head anatomically. Extended retinacular flap development decreases the amount of neck shortening. A short neck increases hip joint instability and decreases abductor lever arm. This approach also allows abductor tensioning to increase joint stability and abductor function.
Stable Slips
Stable slips need stabilization of the epiphysis promptly before the slip can progress or become unstable. Single oblique screw fixation with a large, fully threaded screw stabilizes the slip well.
Physis Assessment Caution
Radiographic lucency in the physis and healed physeal scar may not be easy to differentiate even on CT scan. Dr. Gourineni has had 8 hips opened thinking the physis was still open but found to be fused, leading to 3 major complications.
Healed / Fused Slips
- Healed slips with no OERD do not need treatment.
- OERD amenable to osteoplasty and slip angle <40° — osteoplasty.
- OERD not amenable to osteoplasty or slip angle >40° — corrective osteotomy.
In borderline cases of stable and fused slips, begin with osteoplasty and add an osteotomy if OERD is not fully corrected.
Bone Scan Monitoring
Bone scan within the first few days after surgery identifies head ischemia well. Absent bone scan is of concern and warrants intervention — either immediate exploration or implant removal and MRI at 3 months. The collapse rate has been zero with cold bone scans in stable slips.
Exploration is indicated if the retinacular flap may not be free. This may involve looking for a bone fragment, adjusting the fixation, or Doppler evaluation of the flap. Unstable slips that were cold are the real problems — hips that clot their blood flow after brisk arterial bleeding at the end of the operation are very likely to collapse.
Instability After SSD
Instability of the hip joint after Dunn or neck osteotomy is usually from excessive neck shortening and inadequate G. medius tensioning with GT advancement. Releasing the G. minimus tendon from the GT allows better tensioning of G. medius.
Most slips have normal or anteriorly over-covered acetabulum. Some acetabulae deformed by impingement from the abnormal neck can cause instability. Post-op precautions — maintaining the hip in flexion and abduction for a few weeks until muscle tone returns — prevent anterior subluxation in obese patients. An acetabular osteotomy should be added in hips that are dysplastic and unstable enough that femoral procedures and post-op precautions will be insufficient.
Corrective Osteotomy Options
| Osteotomy | Comments |
|---|---|
| Basicervical | Incomplete correction, shortens neck, shortens abductor resting length, poor cosmetic result |
| Imhauser | Relies on flexion and rotation, leaves trochanter high, no varus correction, deforms femur |
| Southwick | Corrects the head in the socket well, but causes major femoral deformity |
| Modified Dunn | Corrects the deformity at the site — fully corrects, heals rapidly, increases abductor lever arm and resting length, leaves shaft straight. No AVN in Dr. Gourineni's series of 50+ cases for stable slips. |
Modified Dunn Technique — Key Points
The overall technique is the same as standard SSD. Positioning and preparation should use gentle movements because the slip can be or become unstable. GT osteotomy has the risk of getting into the anterior neck from lack of hip internal rotation — a broad, curved osteotome allows a shallower cut anteriorly. The step-cut osteotomy is contraindicated because the GT needs advancement at the end.
The head may be unstable and can displace more during external rotation and dislocation. Blood in the joint, sharp step-off at the head-neck junction, torn anterior retinaculum, and palpable or visible mobility at the physis require pin fixation of the head to the neck prior to dislocation.
Medial callus must be excised before posterior displacement of the epiphysis is possible. A curved osteotome works well to excise medial callus underneath the medial retinaculum. The proximal cut surface should be convex with a 40° angle to the shaft axis and without anterior or posterior tilt. Head reduction that is tight requires sequential shortening of the neck. The head should be centered on the neck in all directions and provisionally fixed with pins, then confirmed with fluoroscopy.
Fixation
Fixation with two implants. Two big screws take away too much epiphyseal bone. One screw and one pin or two–three threaded pins are adequate. Varus alignment of the head increases fixation failure risk; valgus reduction increases instability risk. Anterior tilt of the epiphysis increases hip instability; posterior tilt decreases FIR.
SSD in Healed SCFE (Neck Osteotomy)
Surgical dislocation and retinacular flap development are similar to the Dunn, except the periosteum does not strip easily and trochanteric bone can be harder. In severe slips, there may not be room between the head and posterior GT to develop the posterior flap, so the head must be mobilized by breaking the neck before the posterior vessels are fully protected.
The lateral retinacular vessels (LRV) enter the head at the physeal scar and articular cartilage border. The osteotomy is made 4–5 mm to the cartilage margin, leaving metaphyseal bone attached to the head to protect the LRV at their entry point.
After LRV patency is confirmed, a high-speed burr removes most of the metaphyseal bone from the head and creates concavity to receive the convex end of the neck. Sequential neck shortening may be needed. Leave some metaphyseal bone on the head for better screw fixation. Use two screws with shorter threads to lag the head and allow compression — threads across the osteotomy increase healing problems.