Buckling after Total Knee Replacement
The buckling of the knee after the replacement is instability that may result from preoperative, operative, or postoperative factors. Frequently, instability after total knee replacement requires revision surgery to address the cause of instability and prevent reoccurrence.
Knee replacement surgery consists of replacing the diseased ends of the bones forming the knee joint with metal and plastic prosthetic parts. During the surgery, the surgeon makes bone cuts using templates at specific angles and directions to ensure a balanced artificial knee joint.
Balancing of the knee ensures that there is no excess pressure or tension on the tissues on the front, back, and sides. Additionally, with the balancing of the knee, the deformity is usually corrected, and the body weight is evenly distributed on both sides of the knee joint.
The instability is an early sign of failure of knee replacement. The patients may often complain of knee pain with wobbling or buckling of the knee joint. The risk of instability is greater in patients with connective tissue and inflammatory diseases such as rheumatoid arthritis or diabetes mellitus, etc.
Patients requiring knee replacement surgery often have a preoperative knock knee or bow leg deformity. Patients with bow leg deformity have excessive laxity of tissues on the outer side of the knee while the tissues on the inner side are tight or contracted. During the surgery, the surgeon tries to lengthen the contracted tissues by releasing them from the inner side of the knee.
Similarly, in patients with a knock knee deformity who may need a joint replacement surgery, the tissues on the outer side of the knee are contracted and tight. In order to correct the deformity, the surgeon works to release the tight tissues on the outer side of the knee. Excessive release of tissues on either the inner or the outer side of the knee during replacement surgery may lead to joint laxity and instability.
Patients that may require revision knee replacement due to any reason such as infection, loosening, implant malposition, implant breakage, etc. are also at an increased risk of instability following revision surgery.
After the primary knee replacement surgery, the tissues around the knee joint may often become thick and fibrous. The thickening of the tissues may often make balancing the knee difficult during revision surgery leading to instability. Knee instability after replacement surgery is usually divided/classified depending upon the direction of instability.
Varus-valgus instability (side to side)
The patients experience instability in the side to side direction which may occur as a result of excessive cutting of the lower end of the thigh bone (femur), or inadequate correction of knock knee/bow-legs deformity during the surgery.
The front-back instability may occur as a result of over resection of the back surface of the femur condyle, using a small femur implant, or excessive slope of the tibial implant. In the case of cruciate-retaining (CR) knee replacement, anterior-posterior instability may result from insufficiency of the posterior cruciate ligament.
Mid flexion instability
The instability becomes evident in the form of malrotation on bending the knee from 45 degrees to 90 degrees. A number of different implants and surgical factors may contribute towards mid-flexion instability.
In some patients there may be instability in the form of excessive extension of the leg. The leg overshoots the normal straight alignment of the knee and may become fixed in overextension position as a result of contracture of the iliotibial band. The genu recurvatum deformity may be associated with diseases such as rheumatoid arthritis, Charcot disease, and poliomyelitis.
The instability in multiple planes after knee replacement surgery is known as global instability. Global instability often results from severe bone loss as a result of multiple surgeries.
The diagnosis of instability after a knee replacement is made by the physician based on the history of the patient including the type of surgery and any comorbid conditions. An x-ray, CT scan or MRI may be done to look for implant positioning and the surrounding bone. Blood investigations may be done in case an infection is suspected.
The treatment of knee instability after knee replacement depends upon the degree of instability and the type of instability. The management is usually surgical and bracing may be done in a small subset of patients. The revision surgery may utilize special implants to decrease instability such as hinged implants or implants with rotating platforms.