Mechanical arthritis of the knee For well over a century doctors have recognized a connection between deformities of the lower extremity and arthritis on one side of the knee. Bow legs were associated with arthritis on the medial side of the knee in the man as well as arthritis on the lateral side of the knee in the women with knock knees.
As the idea of disease pathways gained acceptance, a study [Ref.] attempted to connect arthritis of the knee in adults to bow legs and knock knee deformities in infants. The hypothesis was that more aggressive treatment of the infants would prevent the need for later knee replacement.
Fig. A shows a bow leg deformity in a 2 year old child. Fig. B shows a 45 year old overweight adult with arthritis in the medial compartment of both knees. A description of the connection between bow legs and knock knees in infants was first published in 1995. ref This article described what was called the “pendulum system.” This system explained the observation seen in many toddlers that the bow legs and knock knees deformity often behaves like a swinging pendulum finally coming to rest in a vertical position. The toddlers, who have no evidence of rickets, are noticed to have bow legs when they begin to stand. (alignment A). This may correct spontaneously to the correct alignment C or may go on to a knock knee deformity (alignment B). before coming to rest at the correct alignment C. The mechanism must be complete before the age of 5 years otherwise the bow leg or knock knee deformity will persist. The epidemic of obesity has produced a sharp increase in the number of bow legs in children.
Fig x illustrates the changes in the positions of the leg during the “pendulum system” and their potential final positions before the age of 5 years. Early walking and obesity nowadays result in many children over the age of 5 years having a bow leg deformity which requires surgery to straighten out the deformity. Without surgical correction of the deformity the knee will eventually progress on to arthritis and eventual joint replacement. Before the development of Total Knee Replacements the only available treatment for adults with bow legs and arthritis of the medial compartment was to straighten out the leg surgically. This form of late intervention for arthritis-deformed legs became popular in the 1960s and 1970s. The assumption was that the leg straightening out procedure would take the weight off the worn medial side of the knee and place it on the unloaded lateral side. See Fig. x The results, however, were very unsatisfactory. A study of 371 standard realigning operations, called osteotomies, was carried out to determine their outcomes five years after surgery. REF There was an unacceptable number of cases that had become excessively bow legged or knock kneed.
Clearly many patients’ knees were unstable following surgery. These adult cases demonstrate, just as the toddlers showed with the pendulum system, that obtaining and maintaining proper alignment of the knee requires a complex dynamic system based on muscle control. A new minimally invasive mini osteotomy for early intervention in small alignment problems will soon be available. Our studies of how this bow leg/knock knee balancing system works began with this amputee and his prosthesis. The patient shown below has had his left leg amputated above the knee. His prosthesis normally can only flex and extend at the knee. By rotating the prosthesis 90 degrees, however he can duplicate the angulation of bow legs, left photo and knock knees, right photo.
The above illustration demonstrates that a bow leg or knock knee deformity is under the control of specific muscles of the femur. The muscles on the lateral or outer side of the hip can pull the knee away from the body midline, Fig.A . The muscles on the medial or inner side of the thigh can pull the knee toward the midline. Fig. B The femur is maintained in its ideal vertical position during walking by balancing the forces generated by these two sets of opposing muscles. Another type of deformity that causes arthritis in the knee is the so called windswept deformity This less common deformity gets its name from the appearance that a strong wind had been blowing sideways on the legs at the level of the knee. In this condition one limb is knock kneed, while the other is bow legged. The mechanics of the windswept deformity are shown in this graphical reconstruction of the above patient while walking. Light reflectors are placed on various parts of the body and their positions in space recorded on a computer, which reconstructs a picture of the person as a walking skeleton.
Note that the green line (arrows) going from the foot to the center of body weight makes the skeleton’s right knee buckle into a more knock knee position, whereas when the skeleton stands on its left foot that knee buckles into a more bow leg deformity. The windswept deformity pathway begins on the knock knee side. As it progresses over many years it begins to push the opposite knee outward. Eventually both legs need Total Knee Replacements. The need for such bilateral replacement could have been prevented by early surgical realigning of the right, knock knee. Studies performed at Wasserman Motion Analysis Lab. in Baltimore, MD by Anil Bhave, RPT.
Another type of mechanical arthritis of the knee occurs in the medial compartment when a person has long standing arthritis in the hip on the opposite leg. The elderly patient’s right hip illustrated below had developed arthritis when she was young. Over time the hip became shorter and when she walked she leaned over toward the painful hip to relieve the pain. By middle age it was apparent that a bow leg deformity was slowly developing on the patient’s left leg. The right hip and left knee deformity progressed until late in life, when some form of equilibrium was reached between the two joints.
The X-ray of the right hip in Fig. X above shows osteoarthritis (arrow) which is due to the ball of the hip not being kept in its socket during infancy. The arthritis comes from the misfit between the two joint surfaces.
The X-ray of the left knee in Fig. X above shows a bow leg deformity and almost complete destruction of the whole knee joint. This deformity and her consequent arthritis are due to her long standing abnormal gait. Both of the arthritis in the left hip and the subsequent arthritis in the left knee could have been prevented by early intervention of a brace treatment during infancy. These few cases of mechanical based arthritis of the knee demonstrate the critical role that forces and alignment play in the development of preventable arthritis of the knee. With the aid of digital computers, now readily available, it is possible to identify and simulate graphically the different pathways various wear patterns take that culminate in end-stage OA of the knee. The complexity of its mechanical forms indicates that a surgeon-friendly computer program is needed. This would be a valuable tool for those surgeons interested in prevention and early intervention in OA of the knee. Happily, a computer program able to meet this need is expected to be available in the near future. CHEMICAL ARTHRITIS of the KNEE The key to better outcomes for chemical arthritis of the knee begins with early diagnosis. The joint destroying chemicals that are formed in the knee in diseases such as rheumatoid arthritis must be stopped as soon as possible. Much better treatments are available to block the irreversible destruction. Unfortunately early diagnosis is not simple. The multiple chemical signaling pathways of the inflammatory types of arthritis are rapidly being discovered. The conquest of these types of arthritis is definitely on the horizon. |
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