A total knee replacement or arthroplasty is a surgery consisting of replacing the human knee with an artificial knee, often to relieve pain, correct joint deformity and to allow the patient to gain a certain amount of functional independence. A unicompartmental knee arthroplasty follows the same principle, but with only one of the two compartments in the human knee (medial or lateral) being replaced with an artificial one. The latter is usually done on the medial compartment of the knee, seeing as it is more often affected by osteoarthritis. Moreover, a unicompartmental knee arthroplasty allows for a quicker recovery, improved function, as well as less scarring and bruising when compared to a total knee arthroplasty. However, it is more likely to require a revision in 10 years⁴.
In 2010, the prevalence for total knee replacements in the U. S. population was 1.52%, which corresponded to an estimated 4.7 million individuals¹. The prevalence was higher in women than men, and increased with age. Moreover, the demand for total knee arthroplasties is expected to increase over time, with a trend demonstrating a shift toward younger individuals¹. Although total knee replacements have good clinical outcomes, many people still continue to experience significant pain and functional problems after a total joint replacement². Therefore, it is crucial to identify factors or predictors related to poorer outcomes after a total knee replacement.
Why Are Knee Arthroplasties Performed?
Knee arthroplasties are commonly performed because of damage caused by osteoarthritis, rheumatoid arthritis or post-traumatic arthritis³. That being said, identifying the ideal candidate for a total knee replacement is crucial, seeing as patients with OA inappropriately referred for this surgery have an increased risk of poor outcomes⁵.
- Osteoarthritis: cartilage begins to wear away, eventually resulting in pain, stiffness, swelling and bone-on-bone movement due decreased ability of the joint to move smoothly (For more information on knee osteoarthritis, take a look at the following article).
- Rheumatoid arthritis: synovial membrane surrounding the joint becomes inflamed and thickened, damaging the cartilage and eventually causing the loss of cartilage. Consequently, this will also lead to pain, and stiffness.
- Post-traumatic Arthritis: can occur after a serious knee injury (fractures of the bones surrounding the knee, knee ligament tears that may damage the articular cartilage over time, etc.). The end result is typically knee pain and limited knee function.
When Is Surgery Recommended?³
- Severe knee pain or stiffness that limits ADLs/IADLs/transfers (standing up, walking, stairs, etc.)
- Moderate to severe knee pain at rest (day and/or night)
- Knee deformity (varus or valgus)
- Chronic knee inflammation that is not responsive to medical treatment
- Failure to respond to other medical treatments (anti-inflammatories, corticosteroids, lubricating injections, physical therapy, etc.)
Risks and Complications of Knee Arthroplasties
- Infections: patients can present with fever greater than 100 F (37.8 C), shaking chills, drainage from the surgical site or increasing redness, tenderness, swelling and pain in the knee⁶
- Blood Clots (DVTs) and Pulmonary Embolism⁶
- Neurovascular injury³
- Continued pain may be experienced by a small number of patients⁶
- Implant problems such as wear or loosening of device⁶
What Proportion of Patients Experience Long-Term Pain After a Total Knee Replacement for Osteoarthritis?²
A systematic review published in 2012 examined what proportion of patients experience long-term pain after a total knee or total hip replacement for osteoarthritis. For the sake of this article, only the results for the total knee replacement will be presented. The systematic review included eleven studies prospectively examining the proportion of patients experiencing long-term pain after a total knee replacement, for a total of 12 800 patients when pooled. Their results demonstrated that after a total knee replacement, an unfavorable pain outcome was seen in 10-34% of patients. In the studies with the highest methodological quality, the proportion of patients who experienced an unfavorable pain outcome was still approximately 20%.
Predictors of Persistent Pain and Poor Functional Recovery After a Total Knee Replacement
As previously mentioned, many individuals still continue to experience significant pain and functional problems after a total joint replacement². That being said, it is crucial to identify predictors of persistent pain and poor functional recovery after a total knee replacement. Doing so would allow for these predictors to be addressed prior to surgery in order to enhance patient recovery and quality of life.
A recent systematic review and meta-analysis examined predictors of persistent pain after total knee arthroplasty⁷. Thirty-two studies were included in the review, with a total of approximately 30 000 participants when all studies were pooled. Six pre-operative factors were identified as significant predictors of persistent pain after a total knee replacement: greater number of pain sites, higher levels of pre-operative pain, higher levels of catastrophizing, depression, anxiety & poorer levels of pre-operative function. That being said, having had other pain sites prior to surgery, as well as pain catastrophizing, were identified as the strongest independent predictors of chronic, post-operative pain post total knee arthroplasty. This could be explained by the increased attention and awareness of pain exhibited in patients demonstrating pain catastrophizing, consequently magnifying pain intensity. Moreover, increased joint pain and/or having numerous pain sites prior to surgery could indicate a more sensitized nociceptive system⁷.
In addition, recent research further supports the importance of functional mobility as an important determinant of delayed inpatient recovery post total knee arthroplasty, as assessed pre-operatively by both the Timed-Up-and-Go (TUG) and the De Morton Mobility Index (DEMMI)⁸.
Lastly, obese individuals have been identified to be at a higher risk for complications post total knee arthroplasty such as infection, deep infection requiring surgical debridement and revision of the original procedure (exchange or removal of the components for any reason)⁹. Therefore, the latter could also be a potential contributor to poor functional recovery post total knee replacement.
The current scientific literature has identified several predictors of persistent pain and poor functional recovery after a total knee replacement. Addressing these factors is crucial to improve pain outcomes post total knee arthroplasty. Improving pre-operative education of the procedure and outcomes, as well as improving pre-operative functional mobility via weight control/loss, exercise, etc. may significantly improve outcomes post total knee arthroplasty. Moreover, the development and application of effective non-surgical pain management strategies are important, as they may potentially reduce catastrophizing behaviours⁷. Furthermore, evidence suggests that osteoarthritis has an important central sensitization component, which may cause maintenance of persistent nociceptive input from the damaged knee joint¹⁰ ¹¹. Therefore, the timing of these interventions prior to surgery are also crucial, as delaying surgery may also be a risk factor for poor outcomes⁷.
Anthony Teoli MScPT
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