When a patient undergoes a total hip arthroplasty (THA), or hip replacement, a standard restriction is put in place to protect the new joint. This restriction, commonly referred to as the 90-degree rule, requires patients to avoid bending the hip joint more than 90 degrees during the initial recovery period. The purpose of this precaution is to prevent the artificial ball and socket from separating, a complication known as joint dislocation. Adhering to this limit is important in the first six to twelve weeks after surgery while the surrounding muscles and tissues heal and stabilize the implant.
Understanding the Dislocation Risk
Bending the hip past 90 degrees creates a biomechanical situation that can leverage the artificial femoral head out of the acetabular cup, leading to dislocation. This excessive flexion can cause the neck of the femoral component to collide with the edge of the acetabular cup, a process known as impingement. This collision acts as a fulcrum, forcing the ball out of the socket. The risk of mechanical failure is highest when the hip is flexed beyond 90 degrees, especially combined with internal rotation or adduction (crossing the leg past the midline).
The direction of greatest vulnerability depends on the surgical approach used. The posterior approach, a common technique, involves cutting or moving posterior soft tissues, making the joint more susceptible to posterior dislocation. For these patients, the combination of hip flexion past 90 degrees, adduction, and internal rotation poses the greatest risk. Conversely, the anterior approach is associated with a lower risk of dislocation and typically has fewer post-operative restrictions because it is a muscle-sparing technique. However, the 90-degree flexion limit remains a recommended precaution during the early healing phase to protect the joint’s stability.
Immediate Signs of Joint Dislocation
If the hip joint is forced into an unsafe position and the artificial femoral head separates from the cup, the patient will experience a sudden set of symptoms. The most prominent sign is acute, severe pain felt deep in the hip or groin area, occurring immediately upon dislocation. This intense pain is accompanied by the inability to bear weight on the affected leg, making standing or walking impossible.
A visual inspection of the leg will reveal a distinct deformity compared to the unaffected side. In a posterior dislocation, the affected leg will appear shortened, rotated inward, and held in a bent position. For an anterior dislocation, the leg may be rotated outward and abducted. Any attempt to move the leg will be met with muscle resistance and excruciating pain.
Emergency Protocol Following Joint Dislocation
If a hip dislocation is suspected, immediate action is necessary to prevent further harm. The first step is to avoid moving the patient, as any manipulation of the limb could damage surrounding nerves, blood vessels, or soft tissues. The patient should remain in the position they are found in, and a caregiver must call emergency medical services or contact the orthopedic surgical team immediately.
The patient must not attempt to “pop” the joint back into place or allow anyone other than a trained medical professional to do so. In the hospital, the standard treatment is closed reduction, where the orthopedic surgeon manually guides the joint back into the socket. This procedure is typically performed under sedation or general anesthesia to relax the surrounding muscles and minimize pain. Once the joint is reduced, X-rays confirm the correct position of the implant components. A dislocation is a significant event that can interrupt recovery, often necessitating immobilization with a brace or further restrictions to allow stretched tissues to heal and prevent recurrence.
Safe Movement Techniques for Daily Activities
Adhering to the 90-degree restriction requires patients to adopt new movement patterns and utilize adaptive equipment. To prevent excessive hip flexion when sitting, patients should use raised seating, such as an elevated toilet seat or firm, high-backed chairs. When sitting, the knees must remain lower than the hip crease, maintaining an angle greater than 90 degrees.
Bending down to pick up objects must be accomplished using a long-handled reacher or grabber tool, avoiding the need to bend at the waist. If a patient must bend, the recommended technique is the “golfer’s lift.” This involves supporting the body weight on the non-operative leg while keeping the operative leg extended straight behind, maintaining a straight back and an open hip angle. Getting in and out of a car requires backing up to the seat, keeping the knees unbent, and then sliding the entire body into the car to avoid tight angles. Mastering these techniques, often with the guidance of a physical therapist, is fundamental for safely navigating the post-operative period.