TOTAL HIP REPLACEMENT: COMPLICATIONS
Although many complications may follow total hip replacement, their incidence is fortunately low. The most common postoperative complications are deep venous thrombosis, neurologic complications, loosening of prosthetic component, dislocation, fracture, and infection.
The usual risks associated with general anesthesia and stress after any surgical procedure must be discussed with the patient. Because there may be significant blood loss during this procedure, replacement blood must be available. Transient hypotension, an uncommon intraoperative problem specific to hip replacement, occurs in some patients after the compression of cement into the femoral canal. The etiology of this phenomenon is still unclear, but it is theorized that a small amount of unpolymerized monomer in the cement may cause vasodilatation or perhaps the increased pressure in the femoral canal causes fat embolization. Adequate blood volume must be maintained, and rapid infusion of fluids may be required to restabilize the blood pressure.
DEEP VENOUS THROMBOSIS
While not unique to total hip replacement, deep venous thrombosis is unusually common in this type of surgery, occurring in 30% to 50% of cases. Most clots are small and asymptomatic and form in the small veins of the lower limb. Thrombosis of the femoral and iliac veins should be treated early with anticoagulants to minimize the risk of pulmonary embolism. At present, ultrasound is the most common test to screen for DVT, although routine screening is not recommended. Pulmonary embolism serious enough to cause death occurs in a very small percentage of patients, and postoperative treatment with aspirin, low molecular weight heparin, or oral warfarin is therefore recommended. Evaluation of arterial blood gases is required if the patient becomes confused or develops inspiratory chest pain. Although ventilation, perfusion scans, and chest computed tomography (CT) can help rule out a significant pulmonary embolism, angiography is the only definitive test. Fat embolism syndrome from disruption of the femoral bone marrow is rare.
Peripheral nerve palsy is seen in about 0.25% of cases. The femoral nerve can be damaged anteriorly by retractors, and sciatic nerve palsy may occur secondary to retractors, overlengthening of the limb, or pressure from an expanding hematoma. In hip replacement, the most common cause of sciatic nerve palsy is tension placed on the nerve by overretraction or overlengthening of the limb during surgery. This is usually manifested by extensor-evertor weakness of the foot and ankle or decreased sensation in the distribution of the more labile peroneal portion of the sciatic nerve. Although slow improvement usually occurs, complete recovery is uncommon. Rarely, acute dislocation of a total hip replacement causes sciatic nerve injury, necessitating rapid reduction and supportive care.
Loosening of the femoral component is a late complication that usually occurs 15 to 20 years after total hip replacement (see Plate 2-51). Predisposing factors include age (<50 years); weight (>80 kg); and a high level of physical activity. Varus positioning of the femoral stem, poor cementing technique, and certain femoral stem designs are also associated with increased implant loosening. For example, the curved and diamond-shaped femoral stems are more likely to loosen, possibly because of their tendency to fragment the cement. When gaps remain between the bone and the stem of the prosthesis, because of poor cement infiltration or interposition of blood, a piston-like movement can occur and lead to progressive loosening. Porous ingrowth stems have better long-term out-comes, with some stems having a 99% 20-year survivorship when aseptic loosening is used as an end point.
Early loosening can be diagnosed by characteristic radiographic findings even before clinical symptoms appear. Evidence of loosening is a radiolucent zone at the bone-prosthesis, prosthesis-cement, or cementbone interface. Other signs of loosening are subsidence or movement of the components. Thigh pain, the primary symptom of component loosening, is particularly evident when the patient attempts to walk. The pain, which often radiates to the knee, may begin gradually after an initial pain-free interval. Severe pain or rapidly advancing bone lysis indicates the need for revision surgery. Before a revision procedure is undertaken, however, an aspiration arthrogram and culture should be done to rule out infection, which can also cause loosening (see Plate 2-54).
Correction of a loose implant with revision surgery is more difficult than the initial total hip replacement. Careful preoperative planning is most essential. If possible, the previous incision should be used to avoid transecting scars that might compromise the vascular supply and result in skin necrosis. In select cases, a troch slide or osteotomy facilitates the revision procedure by providing the widest possible exposure. All scar tissue and pseudocapsule are excised to allow dislocation and mobilization of the proximal femur. Very loose prostheses can easily be removed. Any remaining debris must be removed carefully in a piecemeal fashion with special instruments. The distal cement plug (if present) is very difficult to remove and must often be penetrated or reamed with a high-speed power burr or hand-operated cement reamer. Long curets, cement chisels, and pituitary forceps are used for deeper access into the femoral canal. Great care must be taken to avoid perforating the femoral canal, because perforations create stress risers that can induce postoperative fractures. If perforation occurs, a femoral component with a long enough stem to reach at least two femoral cortical diameters below the defect must be used for adequate reinforcement.
After the canal is thoroughly cleaned, preparation for a new porous ingrowth component is undertaken.
|FRACTURES OF FEMUR AND FEMORAL COMPONENT|
FRACTURE OF FEMUR
Fracture of the femur can occur around or distal to a femoral prosthesis. Even minor trauma can cause fracture if a significant stress riser is present in the femoral shaft, particularly if the bone is osteoporotic. Any bony defect can act as a concentration point for stress, including screw holes and areas of cortical penetration.
Fractures around a well-fixed component are repaired in standard fashion with open reduction and internal fixation (ORIF) with plates and screws. If the femoral component is loose, the hip is revised to a long-stemmed implant bypassing the fracture by at least two cortical diameters, and the remaining bone is fixed around the new implant with cables.
LOOSENING OF ACETABULAR COMPONENT AND FRACTURE OF ACETABULUM
The incidence of acetabular component loosening increases markedly after 12 to 15 years of function (see Plate 2-53). Improper positioning and bone deficiency above the implant can also accelerate loosening. Although the average rate of polyethylene wear is very low, loosening may result from progressive osteolysis.
Pain in the inguinal region on weight bearing often heralds a loose acetabular component. Radiographs reveal a radiolucent zone around the implant. Because radiolucent zones may be evident before symptoms appear, radiographic follow-up after surgery is very important.
Revision of the loose component involves removal of the loose socket. Large defects in the medial wall of the acetabulum must be repaired with bone grafts. The new acetabular component is then impacted into place.
Segmental defects of the acetabulum are filled with acetabular trabecular metal augments that re-create a spherical acetabulum. A new trabecular metal component is then impacted into the acetabulum and secured to the augment with cement. A fracture of the acetabulum requires revision if function of the prosthesis is impaired. However, sometimes it is best to let the fracture heal first, because the new component can then be inserted more easily. Fractures of the anterior or posterior bony columns of the acetabulum may be repaired with internal fixation and the use of pelvic plates and screws.
DISLOCATION OF TOTAL HIP PROSTHESIS
Dislocation of the prosthesis can occur immediately after surgery if the patient moves the limb into a prohibited position (see Plate 2-53). The patient must avoid extremes of internal rotation, flexion, and adduction for about 12 weeks until a thick capsule forms around the prosthetic joint. Treatment of an early dislocation is immediate reduction with the patient under sedation or general anesthesia. If the components are positioned properly, the patient can resume rehabilitation but must avoid the dangerous limb positions. Use of a hip-hinge abduction brace with a flexion stop is wise because it prevents unguarded movement into the proscribed positions. Recurrent dislocation should be treated according to the cause. If the position of either component is faulty, revision surgery is necessary, or if the myofascial tension is lax, advance osteotomy of the greater trochanter may be indicated when limb length is correct. If the limb is short, the neck of the femoral component may need to be lengthened.