When persistent pain and loss of function from arthritis severely impact daily life, a surgical procedure known as knee arthroplasty, or knee replacement, offers a path to relief. This surgery involves resurfacing the damaged parts of the knee joint with artificial components made of metal, plastic, or ceramic. The two principal approaches are total knee replacement (TKR) and partial knee replacement (PKR), also called unicompartmental knee arthroplasty. The choice between these two procedures depends entirely on the location and extent of the joint damage.
Understanding Knee Compartments
The knee is a complex hinge joint organized into three distinct sections, or compartments. This compartmental structure provides the basis for differentiating between partial and total joint replacement procedures. The primary weight-bearing sections are the medial compartment, located on the inside, and the lateral compartment, situated on the outside.
The third section is the patellofemoral compartment, which involves the joint surface underneath the kneecap (patella) where it glides along the groove in the thigh bone. Total knee replacement resurfaces all three compartments with prosthetic implants to address widespread arthritis. Partial knee replacement (PKR) addresses degenerative changes in only one compartment, leaving the healthy sections untouched.
Scope of the Replacement Procedures
The physical difference between the two operations centers on the amount of bone, cartilage, and soft tissue that is surgically addressed. Partial knee replacement is a less invasive procedure, characterized by a smaller surgical incision. The surgeon targets only the diseased surfaces of the single affected compartment, removing damaged cartilage and a minimal amount of underlying bone to prepare the site for the specialized implant.
A significant benefit of this approach is the preservation of the healthy bone, cartilage, and surrounding ligaments in the two unaffected compartments. Preservation of the anterior cruciate ligament (ACL) and posterior cruciate ligament (PCL) is a common goal of PKR. This bone-sparing technique conserves the native joint mechanics and stability provided by the patient’s own tissues.
Total knee replacement requires a larger incision to allow the surgeon access to all three compartments of the joint. The procedure involves comprehensively resurfacing the ends of the thigh bone (femur) and the shin bone (tibia), and often the back of the kneecap (patella), with larger metal and plastic components. The process involves more extensive bone removal to accommodate the implants and to correct any significant pre-existing deformities. While modern TKR techniques also strive to be bone-sparing, replacing the entire articulating surface means a greater volume of tissue and bone is addressed compared to PKR.
Criteria for Patient Selection
The suitability of a patient for either procedure hinges on the precise location and severity of the arthritic damage. Partial knee replacement is appropriate only for a select group of patients. The primary medical requirement for PKR is that the arthritis must be strictly confined to only one of the three compartments, with the remaining two compartments showing minimal to no evidence of wear or damage.
Additionally, candidates for PKR must meet strict criteria:
- Have healthy, intact ligaments, particularly the anterior cruciate ligament, which is necessary for joint stability.
- Exhibit minimal fixed knee deformity.
- Maintain a good pre-operative range of motion.
- Have an absence of inflammatory arthritis, such as rheumatoid arthritis, which affects the entire joint.
If a patient meets these criteria, the PKR procedure offers advantages.
Total knee replacement is the standard treatment when arthritis has progressed to affect two or all three compartments of the knee. It is also the preferred option when the patient presents with significant pre-existing knee deformity or instability, often resulting from damaged or non-functional ligaments. TKR is a comprehensive solution for widespread damage, providing a stable, durable surface for motion where the native joint structure is no longer viable for a partial repair.
Comparing Recovery and Implant Lifespan
The practical differences in the post-operative experience are important factors for patients considering surgery. Because partial knee replacement is less invasive, it results in less immediate post-operative pain and a shorter hospital stay, sometimes performed as an outpatient procedure. Patients undergoing PKR experience a faster initial recovery, often returning to driving and light daily activities within four to six weeks.
Total knee replacement is a more extensive operation, leading to a longer hospital stay and a prolonged period of rehabilitation. The full recovery timeline often stretches to three to six months for a return to full activity. The greater surgical trauma and larger incision associated with TKR contribute to this extended recovery period and require more intensive physical therapy compared to PKR.
Regarding the longevity of the prosthetic implants, total knee replacements have a well-established track record, often lasting 15 to 20 years or longer. Partial knee replacement implants have historically had a shorter lifespan, though modern designs are improving these outcomes. A primary consideration for PKR is the risk of arthritis progressing in the remaining healthy compartments, which would necessitate a future conversion to a total knee replacement.