Total Joint Arthroplasty: Prosthesis Designs, Biomechanics, and Patient Indications
Key Takeaway
Successful total joint arthroplasty hinges on mastering prosthesis designs, understanding joint biomechanics, and judicious patient selection. Key factors include biomaterial science (metals, polymers, ceramics), fixation principles (cemented, cementless), load transmission, and specific anatomical considerations for joints like the hip. These ensure optimal function, durability, and minimized complications.
A 68-year-old female presents with a 2-year history of worsening left hip pain. She describes a "grinding" sensation and significant morning stiffness. On examination, she has a fixed flexion deformity of 10 degrees and internal rotation limited to 5 degrees. Her radiographs are shown below.
Describe your systematic radiographic evaluation of this patient and outline your pre-operative planning considerations.
Candidate: I would look at the AP pelvis and lateral hip. I see joint space narrowing, subchondral sclerosis, and osteophytes, which is consistent with severe osteoarthritis. For planning, I would check her leg lengths and femoral offset using digital templating to ensure I restore them correctly during surgery. I would also assess the bone quality to decide between cemented or cementless fixation.
Candidates often jump straight to the diagnosis of "osteoarthritis" without a systematic assessment. They fail to mention the importance of the Dorr classification for bone quality, neglect to discuss acetabular version/inclination, and rarely mention the need for a full-length standing scanogram to assess the mechanical axis and true leg length discrepancy.
A structured response is essential: 1. Radiographic Analysis: Evaluate the joint space narrowing, presence of subchondral cysts/sclerosis, and osteophytes. Classify the femoral canal morphology using the Dorr classification (A, B, or C) to guide fixation strategy. 2. Deformity Assessment: Analyze acetabular orientation (inclination and version) and any protrusio or dysplasia. 3. Pre-operative Planning: Use calibrated templating to determine the center of rotation, restoration of femoral offset, and leg length equality. 4. Implant Selection: Justify the choice based on bone quality—cementless for good bone stock (Dorr A/B) vs. cemented for poorer stock (Dorr C). Mention the requirement for a full-length scanogram to rule out distal deformity contributing to apparent leg length discrepancy.
During the procedure, you encounter a sudden longitudinal split in the proximal femur during broaching for a cementless stem. The split is contained within the metaphysis. How do you manage this intra-operatively?
Candidate: I would stop the broaching immediately. I would apply cerclage wires to stabilize the fracture to prevent it from propagating further. Then, I would carefully re-assess the stability of the final stem. If it remains unstable, I would consider a longer stem or switching to a cemented implant.
Failure to mention the Vancouver classification context or the risk of "stress shielding" if a too-long stem is used. Many candidates forget to emphasize protecting the abductors during the cerclage placement or fail to mention the necessity of checking for rotational stability after wiring.
The response should be algorithmic: 1. Containment: Immediately stop reaming/broaching. 2. Stabilization: Secure the fracture with cerclage wiring/cables under tension to prevent propagation. 3. Assessment: Test the primary stability of the prosthesis. If the stem provides good "press-fit" stability despite the fracture, the stem can remain. 4. Escalation: If the stem is unstable, the fracture is deemed "unstable," necessitating a switch to a longer, fluted, tapered stem that bypasses the fracture site by at least two canal diameters, or considering a switch to a cemented construct to achieve immediate fixation. 5. Post-op: Adjust the weight-bearing protocol (e.g., partial weight-bearing) to allow for healing.
