Conversion of Fused Hip to THR : Wheeless' Textbook of Orthopaedics

- Discussion:
- can reduce back and knee pain;
- improved ability to sit for longer periods of time;
- increased incidence of dislocation;
- indications for conversion:
- painful pseudarthrosis
- pseudorthrosis rates are 0-10% using modern techniques for fusion;
- be sure to rule out infection;
- mechanical low back pain - most frequent complaint of patients with fused hips;
- multi-level arthritic changes seen in LS spine 32.9 years following fusion (Amstutz et al, (1975));
- malposition (especially increased abduction) is a major cause;
- excessive leg length inequality also causes LBP over time;
                - Total joint replacement for ankylosed hips. Indications , technique, and preliminary results.
          - knee pain/instability - ipsilateral pain if the hip is fused with increased adduction;
- contralateral pain more likely with increased hip abduction;
- contralateral hip pain - rarely on isolated problem;
- preoperative evaluation:
- ensure that patient has a good indication for conversion
- exam to check skin incisions, leg lengths, nerve function;
- AP pelvis and cross table lateral to identify bone stock, hardware;
- Judet views are helpful to identify anterior or posterior column deficiencies;
- these plain films also help assess the status of the greater trochanter;
- it will sometimes be insufficient due to prior surgery or insufficient growth secondary to pediatric pyarthrosis;
- CT scan can sometimes be helpful for identifying bone stock, the proximity of heterotopic bone to neurovascular structures and
                 abductor muscle mass;
- surgical approach:
- we prefer a standard posterior approach though others report success with a direct lateral or a trans-trochanteric approach;
- we have 3 indications for an trochanteric osteotomy:
- exposure;
- to decrease injury to atrophied/weakened abductors during the case;
- advancement at the end of the case to improve stability
- existing hardware should be removed if it limits exposure or if it is in the way of component placement;
- heterotopic bone should be removed as is possible.
- old hardware can often guide one to the correct plane for bone removal;
- neck cut in situ:
- care must be taken since Amstutz has described 2 eases where cuts were too proximal resulting in fractures of the pubis and the
                        ischium;
- if in doubt, place guide pins and obtain a radiograph;
- soft tissue releases (ilio psoas, adductors, etc.) are frequently required;
- acetabular component:
- structural grafting is sometimes required;
- one should be prepared to cement the socket if the shell has <50 % contact with native bone;
- a constrained liner is frequently required because of insufficient soft tissue tension post-op (esp. abductors);
- femoral component:
- we recommend modular femoral components in order to restore hip mechanics and improve hip stability;
- uncemented stems are sometimes useful if screw holes are present in the proximal femur;
- if cement is used, unicortical screws can be placed in the femur to keep cement in the canal;
- if the trochanter is bald and there are no abductors, the proximal femur can be sewn to the tensor fascia lata anteriorly and te gluteus
                 maximus and ITB posteriorly;
- results:
- relief of low back pain occurs 70-95% of the time
- leg lengths can usually be improved
- ipsilateral knee pain typically improves but it persists in at least 1/3 (especiailly if instability was a problem pre-op)
- a trendelenberg gait typically persists though abductor function improves for 2-5 years;
- results from conversion of spontaneous fusions are typically better than results of conversion of surgical fusions;
- complications:
- deep infection 1.9-15.3% (higher in conversion of surgical fusion)
- dislocation 1.7 - 6.25%
- sciatic nerve palsy 1.8-13.4%
- femoral nerve palsy 3.6%