The last chapter mentioned some of the potential problems that can be associated with specific anatomical features of the knee. Like the hip, some categories of knee problems are broader, and these need specific discussion.
This chapter discusses the most common types of arthritis and other common causes of knee pain, such as meniscal tears, bursitis, osteochondral lesions, and avascular necrosis. Another important cause of knee pain is actually hip joint problems; hip arthritis commonly can present as knee pain. It is not unusual to see a patient who reports they have been told x-rays of his knee are fine, but that he has serious knee pain and limitation, only to find a hip x-ray shows significant pathology in his hip!
Osteoarthritis of the Knee
Osteoarthritis, or the "wear and tear" variety of arthritis, is probably the most common reason for knee pain in mature adults. It is certainly the problem that I see most commonly with patients aged 50 or older who report pain that has been present for longer than several months, although it is by no means the only cause for such pain. It also can be seen in patients of much younger age, especially in obese patients.
The articular cartilage is a coating of smooth, soft cartilage about 1/8th inch thick covering the ends of the femur, tibia, and patella undersurface. As mentioned in the last chapter, it can wear away with time or injuries, and after skeletal maturity it does not grow back.
There definitely appears to be a genetic predisposition to developing osteoarthritis.
Frequently, one of the compartments of the knee (usually the medial, or inner side of the knee) may wear out sooner than the other compartments. In this case, some options are available for treatment that are not useful when all three compartments are affected, such as orthotics, off-loader braces, and partial knee replacements. These options for single compartment disease will be discussed more fully in the next couple of chapters.
As the cartilage covering begins to wear away, underlying bone surfaces are eventually exposed (hence the common term "bone on bone" arthritis). As the process continues, the body responds by making osteophytes (large spurs) around the peripheral edges of the joint and often by making more joint fluid. This can result in a large joint effusion, or "water on the knee," which sometimes needs to be drained. The knee will also usually become progressively more stiff as the process continues, and frequently an angular deformity may develop as one side wears out faster than the other. This commonly results in a varus (or "bow-legged") appearance, although sometimes it can result in a valgus (or "knock kneed") deformity. A flexion contracture may eventually develop in which the knee no longer can fully straighten out.
Trauma And Post-Traumatic Arthritis
There are numerous traumatic injuries that can occur in the knee, ranging from contusions and bone bruises to fractures involving the joint surface or nearby shafts of the tibia or femur. Trauma can result in injury to any of the ligaments of the knee or even to multiple ligaments, each requiring treatment that varies from simple bracing and observation (such as a partial medial collateral ligament injury) to extensive reconstruction surgeries (such as an ACL reconstruction). There is a wide variation in treatments of acute injuries, and we will not be able to discuss those in detail here as this book is primarily dedicated to joint replacement and related surgeries.
However, it is important for our discussion here to understand that post-traumatic arthritis can develop after injuries to the knee, and clinically it looks and is treated similar to osteoarthritis. It is not uncommon for patients who have had prior traumatic injuries to the knee to develop post-traumatic arthritis years later. A large percentage of patients who sustain tibial plateau fractures (fractures that involve the weightbearing surface of the tibia) require knee replacement surgeries in the years after the injury, despite adequate bone healing with plates and screws after the initial injury. The smooth surface of the joint becomes disrupted, and over time arthritis results.
For many years, the standard treatment for meniscal tears involved resecting the entire meniscus from the affected side of the knee. However, it turns out that the meniscus serves an important function as a stabilizer for the knee, and without it most people develop severe arthritis in the knee over the years after excision. After the development of arthroscopic surgery, complete meniscectomies became rare, and most surgeons treat nonhealing meniscal tears with arthroscopic surgery in which only the torn portion of the meniscus is removed (and sometimes repaired, if possible). However, it is common to see patients who had complete meniscectomies (or removal of "torn cartilage in the knee") years ago who now require partial or total knee replacement.
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