The lower limb Mechanical Axis is defined by a line running through the centre of the hip joint to the centre of the ankle joint. This line should pass essentially through the centre of the knee joint.
The tangential distance of this line from the centre of the knee joint is referred to as the Mechanical Axis Deviation.
In a Varus knee, the axis passes through the knee medial compartment, while in a Valgus knee it crosses the knee lateral side.
Another key concept is the one of Anatomical Axis. The Anatomical Axis of a bone is represented by a line drawn down the centre of the medullary cavity of the bone.
In the femur, the mechanical and anatomical axes deviate whereas in the tibia the mechanical and anatomical axes are essentially represented by the same line.
The angle between the Anatomical Axis of the femur and the lower limb Mechanical Axis is normally 6° of Valgus.
Last week I was invited to attend in a knee surgery at the Hospital Cavale Blanche, in Brest. For my PhD project, the research work that I am currently carrying on is actually closer to the mechanical side of Robotics (microactuators, tiny mechanical structures, wireless energy transfer techniques …). Although, since one of my supervisors is an orthopedic surgeon, I was invited to participate to TKA surgery. Basically, this meant two things: I had to put on specific sterilized clothes and, once in the operating room, I was allowed to watch and put questions 🙂
When I got here I was told that, for PhD students in the field of Robotic Surgery, half a day at the hospital was more useful, in terms of acquirable knowledge, than a month of theoretical study. Actually, one of the aspects I mostly appreciated was the “pedagogical approach” of the surgeon towards the observers. We were two, a sweet girl who’s completing her studies in Pharmacology and me. And the surgeon kept explaining each single step, motivating each procedure and answering all our questions. The confidence he showed while (literally) manipulating the patient’s knee and conducting the operation in front of younger and less experienced surgeons was simply amazing.
Before the surgery, the patient can decide to undergo either general or local anesthesia. Let’s say that the noises and, especially, the smells produced by hammers and saws are not the best way to stimulate one’s appetite 😉 But one thing is sure: after living the atmosphere of such a delicate surgical operation, and understanding that one can really trust expert surgeons, personally I wouldn’t be so worried/scared to undergo TKA as I was before 🙂
Medically speaking, the “cartilage” is actually known as the meniscus. The meniscus is a C-shaped piece of fibrocartilage which is located at the peripheral aspect of the joint. The majority of the meniscus has no blood supply. For that reason, when damaged, the meniscus is unable to undergo the normal healing process that occurs in most of the rest of the body. In addition, with age, the meniscus begins to deteriorate, often developing degenerative tears. Typically, when the meniscus is damaged, the torn piece begins to move in an abnormal fashion inside the joint.
Because the space between the bones of the joint is very small, as the abnormally mobile piece of meniscal tissue (meniscal fragment) moves, it may become caught between the bones of the joint (femur and tibia). When this happens, the knee becomes painful, swollen, and difficult to move.
The meniscus has several functions:
Stability – As secondary stabilizers, the intact meniscii interact with the stabilizing function of the ligaments and are most effective when the surrounding ligaments are intact.
Lubrication and nutrition – The meniscii act as spacers between the femur and the tibia. By doing so, they prevent friction between these two bones and allow for the diffusion of the normal joint fluid and its nutrients into the tissue which covers the end of the bone. This tissue is known as articular cartilage. Maintenance of the integrity of the articular cartilage is critical to preventing the development of post-traumatic or degenerative arthritis.
Shock absorption – The biconcave C-shaped pieces of tissue known as meniscii (cartilage in non-medical terms) lower the stress applied to the articular cartilage, and thereby have a role in preventing the development of degenerative arthritis.
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