Avascular necrosis of the femoral head is a serious condition which can rapidly lead to the destruction of the femoral head subchondral bone and the overlying cartilage, ultimately resulting in secondary hip arthritis, potentially at a young age. Core decompression has been described as a potentially joint sparing procedure for patients with osteonecrosis.
The rationale behind this procedure is to stimulate the surrounding area of healthy tissue to produce new bone by drilling through the necrotic zone. Drilling acts as the necessary stimulus to initiate a healing response from the viable bone cells of the surrounding areas, which in turn migrate to the necrotic site and gradually replace the necrotic bone with healthy one. Furthermore, the addition of viable bone precursor cells from the patients’ own bone marrow, taken during the procedure and delivered to the site through the drill holes, further enhances the regional healing response. Since new bone formation can only replenish the extent of the necrotic area but can’t restore the shape of the femoral head, core decompression is only effective at the stages of the disease before collapse of the femoral head occurs.
As with every other procedure, preoperative checks have to be run before the patient is cleared for surgery. Another important parameter prior to decompressing the femoral head is the three-dimensional assessment of the necrotic area so as to plan the precise trajectories of the drillings. This is achieved through the detailed interpretation of the pateints’ radiographs, computed tomography scans and magnetic resonance imaging by the operating surgeon.
General anaesthesia is used for the majority of cases and the patients is placed in the supine posiiton on a radiolucent traction table. This is a specialised operating table with a well-padded perineal post placed between the patietns’ legs and traction boots applied to one or both lower limbs, which permit placing them in traction in a controlled fashion. Throughout the procedure an image intensifier is used to obtain radiographs of the operative hip in various positions, as well as guide the drills in order to precisely aim the area of the osteonecrosis. As such, either a small incision all the way to the bone or three to four stab incisions are made on the lateral aspect of the thigh, at the desired level. Under image intesification, guide-wires are passed through the lateral cortex aiming for the necrotic zone. Various techniques have been described regarding the number, diameter and location of the drills used, with some authors speaking of one centrally placed, gradually expanded to a large diameter drill hole, while others prefer the use of two to three (depending on the size of the necrotic area), smaller diameter drill holes. In any case, each trajectory is overdrilled two to three times to ensure adequate healing response stimulation. As previously mentioned, bone marrow aspirate can be obtained prior to the beginning of the procedure, which after centrifugation and, possibly, the addition of demineralised bone matrix, can be injected into the femoral head. Following this, some surgeons may also inject calcium phosphate, a bone substitute, to seal the drill trajectories and avoid spillage of the bone marrow aspirate. Closure of the wound is performed in a layered manner.
As is the case with every surgical procedure, core decompression can lead to certain complications. Wound infection rates are generally low (approx. 1%) due to the minimal soft tissue trauma, especially with the percutaneous methods, and the use of periopertive prophylactic antibiotcs. Perioperative prophylaxis is also administered for venous thromboembolism (i.e. deep venous thrombosis and pulmonary embolism) in the form of injectable low molecular weight heparin. Iatrogenic femoral neck fractures and subtrochanteric fractures are rare complications with few reports of them. Measures taken to avoid them include the use of as few drill trajectories as possible to cover the necrotic area and the avoidance of drilling distal to the level of the lesser trochanter.
Postoperatively, patients are instructed to touch-down weight-bear for four to six weeks, followed by partial weight bearing for another three to four weeks and progession to full weight bearing after the two month milestone. Supervision by a skilled physical therapist beginning immediately after the operation is imperative. The bone reparative effect induced by core decompression can be assessed radiographically from six weeks postoperatively and onward at six to eight week intervals, until the sixth postoperative month. Success rates of core decompression can vary greatly among patients with osteonecrosis and are greatly dependent upon the stage of the disease (i.e. whether it has affected or not the subchondral bone and the shape of the femoral head), the location and the size of the necrotic area. Patients at pre-collapse stages, with centrally (rather than laterally) located necrosis and combined angle of the necrotic area measuring less than 240° can expect almost 90% success at healing of the necrotic area with core decompression.
Apart from the use of autologous bone marrow aspirate which is readily available from the patients’ own pelvis during the operation, other factors have also been used to enhance the bone healing response to the core decompressions. These include bone graft from the patient or from a cadaveric donor, bone morphogenetic proteins and even stem cells. Though their efficacy has been reported on small patient series, larger randomised, controlled trials are needed to further validate it. Finally, robotic arm – assisted drilling rather than the original, free-hand technique has also been reported.
In conclusion, core decompression is a joint-sparing procedure for early stage osteonecrosis of the femoral head, which works through stimulating new bone formation within the necrotic area by means of drilling. It is a relatively safe and simple procedure carried out under general anaesthesia, which is associated with a low risk of complications. Postoperative rehabilitation can be lengthy given the patients’ already undermined by the osteonecrosis itself functional status at the time of the operation. However, when performed in approprietly selected patients, core decompression can yeald significantly favourable outcomes and delay or even obviate the need for hip arthroplasty, especially at a young age.
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