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This page from University of Washington on replacement and tendon repair says:
For example, replacement joints cannot withstand heavy lifting, so we would recommend that patients avoid such activity even if they could otherwise pursue arduous physical tasks.
Why can't artificial tendons withstand heavy lifting?
I am sensing some mix up here.
Your are asking about artificial tendons but your quote is stating about joint replacements.
Basically your equating apples and oranges.
Joint replacements or joint arthroplasties are used to treat severely destructed joints. In four cases of five the reason for joint destruction is osteoarthrosis in which the joint cartilage has worn off resulting to pure bone to bone contact which is very painful and disabling.
During joint replacement surgery the severely damages joint surfaces are sawed off and they are replaced with identically shaped components. These component usually have two options for fixation. They can be cemented, in which polymethylmethacrylate (PMMA) is put in both components. Then components is compressed to molded end of the bone and cement penetrates to porous bone tissue. The better and deeper the cement penetration, the firmer is the fixation. Another option is to use hydroxyaphatite (HA) coated components. Component is again compressed against bone, but the fixation is achieved by bone ingrowth to to HA coating.
Bearing material is also important aspect in joint replacements. Knee and hip are by far the most commonly replaced joint. In hips there is usuallu metallic head bearing agains cup made from polyethylene (PE). In knee both komponents are usually metallic but there is an insert made from PE between components to reduce friction.
Fixation and bearing materials are the weak links in joint replacements. Weight lifting or heavy exercises are not recommended in patients with replaced joint. Most importantly wear is always present in each and every joint replacement, current medicine does not know a bearing material which does not wear. Wear of PE correlates with use. The more you put weight or strain to your hip/knee replacement the more it wears and more likely it will result to complications associated to increased release of PE particle.
Intense repetitive movement in the joint can also cause cement debonding or breakage in the bone-HA interface and to loosening of the components.
Hip or knee are so complex mechanical systems that each and every movement or momentum can not be handled with current knowledge regarding mechanical aspects joint replacements. At this time joint replacements tolerate very well walking, daily activities and light weight training.
Of course nowadays we can use all ceramic or all metallic bearings but I won´t dig in to those. Use of PE is so common and traditional in joint replacements that these restriction quoted by OP are important to >90% of patients. Tour de France winner Floyd Landis for example has all metallic hip replacement.
Artificial tendons are another thing. Treatment of ruptured anterior cruciate ligament (ACL) is very common procedure these days. Most commonly ACL is replaced with autograft. That means a tissue retrieved from another site in the SAME patient.
In ACL reconstruction common site for tendon autograft is hamstring or gracilis tendon graft. This graft is retrieved with special instruments, cut to required length ja then fixed with bioabsorbable screws to femur and tibia. With time the tendon provides perfect ingrowth to the bone since the tendon is retrieved from same patient and there is no issue regarding tissue rejection.
Usually young patient with ACL rupture can return to heavy exercise without any restrictions after successful ACL reconstruction.
The cite relates to joint replacement, a procedure of removing the painful joint surfaces and replacing them with an artificial joint (usually made of a soft synthetic material) which are placed in the ends of the bone – re-creating the joint that has been removed. The artificial surfaces of the joint replacement are shaped in such a way as to allow joint movement similar to that of a healthy and natural joint.
Sports or occupations requiring repetitive overhead motion or heavy lifting can place a significant strain on rotator cuff muscles and tendons. Over time, as a function of aging, tendons become weaker and degenerate which can lead to complete tears of both muscles and tendonsNIH.
Other risk of heavy lifting (sudden trauma on the joint) can include shoulder dislocation as the result of loosening of the components (between the bone and the components) and weakening the muscles around the joint causing the ball of the prosthesis come out of its socket. In most cases, the hip can be corrected without surgeryNIH, wiki.
