Half Hip Replacement (Hemiarthroplasty): Procedure, Recovery, and Risks

What is a Half Hip Replacement?

A half hip replacement, medically known as a hemiarthroplasty, is a surgical procedure that involves replacing only the femoral head (the "ball" part of the hip joint) with a prosthetic implant, while leaving the natural acetabulum (the "socket" part) intact.

Introduction to Hemiarthroplasty

A half hip replacement, or hemiarthroplasty, is a common orthopedic procedure distinct from a total hip replacement. While both aim to alleviate pain and restore function to a damaged hip joint, hemiarthroplasty specifically addresses damage or fracture to the femoral head, preserving the natural acetabular cartilage. This targeted approach is often preferred for certain patient populations and injury types, particularly acute femoral neck fractures in older adults.

Anatomy of the Hip Joint

To understand a half hip replacement, it's crucial to grasp the basic anatomy of the hip. The hip is a ball-and-socket joint, one of the body's largest weight-bearing joints, designed for extensive mobility and stability.

• Femoral Head: The "ball" is the rounded upper end of the femur (thigh bone).
• Acetabulum: The "socket" is a cup-shaped indentation in the pelvis.
• Articular Cartilage: Both the femoral head and acetabulum are covered with smooth, slippery articular cartilage, which allows the bones to glide effortlessly against each other during movement.
• Synovial Fluid: Lubricates the joint, further reducing friction.

In a hemiarthroplasty, only the femoral head is replaced, meaning the natural acetabular cartilage must be healthy enough to articulate with the new prosthetic ball.

Components of a Half Hip Replacement (Hemiarthroplasty)

The prosthetic components used in a hemiarthroplasty typically consist of two main parts:

• Femoral Stem: This component is inserted into the hollow center of the femur. It can be cemented into place or designed for "press-fit" fixation, relying on bone ingrowth for stability.
• Femoral Head (Ball Component): This ball-shaped component is attached to the top of the femoral stem. It is typically made of metal (e.g., cobalt-chromium alloy) or ceramic and articulates directly with the patient's natural acetabular cartilage.
  • Unipolar Hemiarthroplasty: Features a single femoral head that directly articulates with the acetabulum.
  • Bipolar Hemiarthroplasty: Features a small inner head that articulates within a larger outer head, which then articulates with the acetabulum. This design aims to reduce wear on the natural acetabular cartilage, though its long-term benefits over unipolar designs are debated.

Crucially, no artificial socket is implanted into the pelvis during a hemiarthroplasty.

Indications for Hemiarthroplasty

Hemiarthroplasty is primarily indicated for specific conditions where the damage is localized to the femoral head, and the acetabulum remains healthy. The most common indication is:

• Femoral Neck Fractures: Particularly in older adults, where the fracture disrupts the blood supply to the femoral head, leading to avascular necrosis (bone death) or significant displacement that is unlikely to heal well.
  • It is often preferred over open reduction and internal fixation (ORIF) for displaced femoral neck fractures in elderly patients due to lower rates of re-operation and better functional outcomes.
• Less Common Indications: Occasionally, it may be considered for severe avascular necrosis of the femoral head without significant acetabular involvement, or in select cases of hip osteoarthritis where the acetabular cartilage is surprisingly well-preserved relative to the femoral head damage.

How Hemiarthroplasty Differs from Total Hip Replacement (THR)

Understanding the distinction between hemiarthroplasty and total hip replacement (THR) is vital:

• Hemiarthroplasty: Replaces only the femoral head. The patient's natural acetabulum (socket) is preserved.
• Total Hip Replacement (THR): Replaces both the femoral head and the acetabulum. An artificial socket is implanted into the pelvis, and a new femoral head and stem are implanted into the femur.

Key Differences and Implications:

• Complexity: Hemiarthroplasty is generally a less extensive surgery than THR, potentially leading to shorter operative times and less blood loss.
• Indications: Hemiarthroplasty is primarily for acute fractures with a healthy socket, while THR is typically for chronic conditions like severe osteoarthritis or rheumatoid arthritis that have damaged both sides of the joint.
• Longevity and Wear: With hemiarthroplasty, the prosthetic femoral head articulates directly with natural cartilage, which can eventually wear down, potentially necessitating a conversion to THR in the future. THR involves two artificial surfaces articulating, designed for long-term wear resistance.
• Stability: Both procedures aim for stability, but dislocation risk and other mechanical considerations can differ based on surgical approach and patient factors.

Surgical Procedure Overview

A hemiarthroplasty is performed under general or spinal anesthesia. The surgeon makes an incision (commonly posterior, lateral, or anterior) to access the hip joint. The damaged femoral head is removed, and the femoral canal is prepared to receive the prosthetic stem. The stem is then inserted into the femur, and the new prosthetic femoral head is attached. The joint is reduced (put back into place), and the incision is closed.

Recovery and Rehabilitation

Post-surgical rehabilitation is critical for optimal recovery and restoration of function following a hemiarthroplasty.

• Early Mobilization: Patients are typically encouraged to begin moving and walking with assistance very soon after surgery, often within 24-48 hours.
• Physical Therapy (PT): A structured PT program is essential. It focuses on:
  • Pain Management: To allow for participation in therapy.
  • Range of Motion (ROM): Restoring normal joint movement, while respecting any post-operative precautions (e.g., avoiding extreme flexion or internal rotation for posterior approaches to prevent dislocation).
  • Strengthening: Targeting key muscles surrounding the hip, including the quadriceps, hamstrings, gluteal muscles (gluteus maximus, medius, minimus), and core stabilizers, which are vital for gait and balance.
  • Gait Training: Re-learning to walk properly with appropriate weight-bearing progression.
  • Balance and Proprioception: Exercises to improve stability and body awareness.
• Activity Progression: Gradual return to daily activities, including walking, climbing stairs, and light household tasks, is guided by the physical therapist and surgeon. High-impact activities may be restricted.
• Home Exercise Program: Patients are provided with exercises to continue independently at home to maintain progress.

Potential Risks and Complications

While generally safe, like any major surgery, hemiarthroplasty carries potential risks:

• Dislocation: The prosthetic femoral head can come out of the natural acetabulum.
• Infection: At the surgical site or within the joint.
• Deep Vein Thrombosis (DVT) / Pulmonary Embolism (PE): Blood clots in the leg or lungs.
• Nerve or Vascular Damage: Though rare, injury to surrounding nerves or blood vessels can occur.
• Leg Length Discrepancy: One leg may feel longer or shorter after surgery.
• Periprosthetic Fracture: A fracture occurring around the prosthetic components.
• Aseptic Loosening: The implant becoming loose from the bone over time without infection.
• Acetabular Cartilage Wear: Over time, the natural cartilage in the acetabulum can wear down due to articulation with the prosthetic head, potentially leading to pain and the need for conversion to a total hip replacement.

Outcomes and Longevity

Outcomes for hemiarthroplasty are generally favorable, particularly for the target population of older adults with femoral neck fractures. Patients often experience significant pain relief and improved mobility, allowing for a return to functional independence. The longevity of a hemiarthroplasty can vary. While the femoral component itself can last many years, the primary concern for long-term durability is the wear of the natural acetabular cartilage. If this cartilage wears out, it can cause pain and necessitate a revision surgery to a total hip replacement.

Conclusion

A half hip replacement, or hemiarthroplasty, is a specialized orthopedic procedure that effectively addresses damage to the femoral head while preserving the natural hip socket. It is a critical intervention, particularly for displaced femoral neck fractures in older individuals, offering a pathway to pain relief and restored mobility. Understanding its specific indications, components, and differences from total hip replacement is key for both patients and healthcare professionals in navigating hip joint pathology and rehabilitation.