How Joint Replacement Works: Hip, Knee Surgery, Recovery, and Outcomes

This article is for informational purposes only. Consult a qualified healthcare professional for medical advice, diagnosis, or treatment.

What Is Joint Replacement Surgery?

Joint replacement surgery—medically termed arthroplasty—is a procedure in which a damaged or arthritic joint is removed and replaced with an artificial implant (prosthesis) designed to replicate the mechanical function of the natural joint. Total knee arthroplasty (TKA) and total hip arthroplasty (THA) are among the most performed elective surgical procedures in the world. In the United States alone, approximately 790,000 total knee replacements and 450,000 total hip replacements are performed annually, and these numbers are projected to grow dramatically with aging populations.

Indications and Patient Selection

The primary indication for joint replacement is end-stage osteoarthritis—the progressive breakdown of articular cartilage that leads to bone-on-bone contact, severe pain, and loss of function. Other indications include rheumatoid arthritis, avascular necrosis (death of bone tissue due to interrupted blood supply), post-traumatic arthritis, and fractures in elderly patients (particularly femoral neck fractures for hip replacement).

Key criteria for proceeding to surgery include:

Moderate-to-severe persistent joint pain significantly affecting daily activities and quality of life
Radiographic evidence of severe joint space loss
Failure of conservative treatments (NSAIDs, physical therapy, corticosteroid injections, bracing) over several months
Patient motivation and capacity for rehabilitation

Implant Materials and Design

Modern joint implants are engineered for durability, biocompatibility, and biomechanical accuracy. They consist of multiple components:

Component	Material	Function
Femoral component (knee) / femoral head and stem (hip)	Cobalt-chromium alloy or titanium alloy	Replaces the hard bone surface; smooth articulating surface
Tibial tray (knee) / acetabular cup (hip)	Titanium alloy with porous coating	Fixed to bone via cementless press-fit or bone cement; provides base
Bearing insert (liner)	Ultra-high molecular weight polyethylene (UHMWPE), ceramic, or metal	Articulating surface between components; absorbs compressive force
Patella resurfacing (optional, knee)	Polyethylene button	Replicates the back surface of the kneecap

Cementless fixation uses porous-coated or hydroxyapatite-coated implant surfaces that allow bone ingrowth (osseointegration) over 6–12 weeks. Cemented fixation uses acrylic bone cement (polymethylmethacrylate, PMMA) for immediate fixation—often preferred in elderly patients with osteoporotic bone.

Total Hip Replacement: Surgical Technique

The degenerative femoral head and the damaged acetabular cartilage are removed. The acetabular socket is reamed to accept the metal cup, which is then press-fitted or cemented into place and typically supplemented with screws. A polyethylene, ceramic, or metal liner is inserted into the cup. The femoral canal is prepared with rasps to the exact geometry of the femoral stem, which is then inserted and secured. The femoral head component—a ball—is attached to the stem. The new ball is then reduced into the cup. Soft tissue tension and implant stability are tested through range-of-motion examination before wound closure.

Total Knee Replacement: Surgical Technique

Precise bone cuts are the foundation of knee replacement. Using intraoperative guides (mechanical, computer-assisted, or robotic), the surgeon removes measured slices of bone from the distal femur, proximal tibia, and sometimes the back surface of the patella. The metal femoral and tibial components are sized to these cuts and trialed with polyethylene inserts to test stability, alignment, and flexion-extension gap balancing. Once optimal size and position are confirmed, final components are cemented or press-fitted into place. Robotic-assisted systems (Mako, Rosa) improve precision of bone cuts and implant positioning, with some evidence of improved early outcomes.

Anesthesia Options

General anesthesia: Patient unconscious; commonly used historically.
Spinal (subarachnoid) block: Numbs from the waist down; increasingly preferred as it reduces blood loss and postoperative nausea compared with general anesthesia.
Nerve blocks: Regional nerve blocks (femoral, adductor canal, sciatic) provide targeted postoperative analgesia, reducing opioid requirements.

Recovery Timeline

Milestone	Typical Timing
Standing and walking with assistance	Day 0–1 post-op (same-day or next-day mobilization is now standard)
Hospital discharge	1–3 days for TKA; often same-day or 1 day for THA at high-volume centers
Walking without aids (cane or walker)	4–6 weeks
Driving (automatic vehicle)	4–6 weeks after right-side surgery
Return to sedentary work	2–4 weeks
Return to physical work	3–6 months
Full functional recovery	3–12 months (knee typically longer than hip)

Long-Term Outcomes and Implant Lifespan

Modern implants are exceptionally durable. Data from national joint registries show that approximately 90–95% of total knee replacements and 95% of total hip replacements are still functioning at 10 years. Implant survival at 20 years is approximately 80–85% for TKA and 85–90% for THA in primary surgeries. Revision surgery—replacing a failed implant—is more complex and has lower success rates, making initial optimal implant positioning and patient selection critical.

The vast majority of patients report dramatic pain relief and functional improvement. Studies consistently show 85–90% patient satisfaction rates, with patients returning to recreational activities such as walking, cycling, swimming, and golf. High-impact activities like running on hard surfaces are generally discouraged to minimize implant wear.

Risks and Complications

All major surgeries carry risk. Specific concerns in joint replacement include deep vein thrombosis (DVT) and pulmonary embolism—addressed with anticoagulation and early mobilization; periprosthetic joint infection (PJI), which affects approximately 1–2% of primary replacements and is devastating if not promptly treated; implant loosening over time; and instability or dislocation, particularly in the first few months after hip replacement. Metal hypersensitivity reactions to cobalt-chromium can rarely cause local tissue necrosis (pseudotumor), particularly with metal-on-metal bearings that have largely been abandoned.