An artificial replacement of a ball and socket hip joint can restore function to a hip compromised by injury or arthritis. The procedure is generally successful. However, during total hip replacement surgery, your doctor cuts through the ligamentous capsule and other soft tissues that surrounds and helps center the ball of the joint in its socket. This reduces the stability of the hip joint and increases the risk of dislocation.
Certain surgical approaches during hip replacement increase the risk of dislocation. Prosthetic components may lack proper orientation or be grossly malpositioned. Certain designs may be inappropriate and cause components to lever out against bony prominences. Previous hip surgeries lead to laxity of the soft tissues. Post-surgical fractures can also cause a hip dislocation. Following hip replacement surgery, as the tissues are healing, your doctor gives you specific instructions on avoiding certain movements to prevent a dislocation. These include:
- Not bending the hip more than 90 degrees: You must avoid sitting on low sofas, instead use high chairs or beds that have been raised. While picking an object off the floor you must extend the operated leg backwards while supporting yourself on a piece of furniture.
- Not turning your knee inwards: You must not cross your knees while sitting and when lying on your side, instead raise your knee with a pillow to prevent it from falling inwards.
These precautions are compulsory for at least 6-12 weeks after the surgery so that there is minimal stress on the healing soft tissues and a thick capsule may form around the joint. Non-compliance to these instructions increases your risk for dislocations.
Artificial hip dislocation is rare but is likely to occur within the first 3 months after surgery. You may experience pain, a popping sound, inability to bear weight or move your leg/hip completely, a feeling that the hip has slipped out of its joint, or that the affected leg is shorter or turns inwards. X-rays are obtained to evaluate component orientation, malposition and wear of the components as well as soft tissue wear and nonunion. Bone and marrow samples may be obtained and sent to the laboratory to check for infection.
Early dislocations (before 6 weeks) usually occur due to relaxed soft tissues and immature scar tissue formation and are treated with bed rest followed by application of a brace or cast until the tissues strengthen. The dislocated artificial hip is usually treated without surgery by externally manipulating the ball back into the socket (closed reduction).This is usually accomplished under anesthesia, by stretching, traction and mobilization maneuvers. Medication may be administered to relax the hip muscles. As the procedure is painless, your doctor is able to use enough force to break up scar or soft tissue adhesions that have formed around the joint and separate the components before reducing them into correct position.
If joint dislocations become frequent, a revision surgery may be necessary. Revision surgery may involve the following:
- Removal of the misaligned prosthetic components and replacing them in proper anatomical position.
- Hardened cement that was used to fix the prosthesis has to be carefully removed to avoid a bone fracture. The freed components are then aligned in proper position.
- Soft-tissue balance may be restored by repositioning the soft tissues to provide adequate tension.
- Any impinging soft tissue, bone or cement is removed.
- As there is usually bone loss around the prosthetic component, your doctor may use a bone graft taken from your own body or a bone bank to reinforce the bone.
- If a fracture occurs, the fracture is repaired as part of the procedure.
- If infection is present, you will require a replacement of your prosthesis.
- In certain cases where dislocations are recurrent and no clear cause exists, a prosthesis that constrains your movement may be beneficial.
It is necessary to follow the precautions given and adhere to the rehabilitation program to maximize hip function and avoid future hip dislocations.