Abstract
Currently available knee prostheses can provide 100 to 110° of knee flexion and this is generally good enough to ascend and descend stairs, arise from a chair, and perform most of the daily life activity. However, in certain situations like gardening, sitting on the flat floor and activities that require a squatting position, deep knee bends are required. In some countries, such as Japan, deep knee flexion is very important for the activity of daily life such as leading a life on a Tatami mattress and using a Japanese style toilet. There are several crucial factors, which influence postoperative knee flexion. Those are 1.) preoperative range of motion, 2.) surgical technique, 3.) prosthesis design, and 4.) postoperative rehabilitation.
If a patient has longstanding, poor, preoperative range of motion, then the extensor mechanism itself became stiff in addition to the periarticular fibrotic change of the soft tissue and severe destruction of the bony structure. In this circumstance, it is awfully difficult to obtain deep knee flexion with currently available prostheses and surgical techniques. This indicates that we cannot wait for the last minute to perform TKR if a patient desires to gain deep knee flexion after the surgery.
Surgical technique influences postoperative range of motion significantly. Anatomically the structures that get tight in knee flexion are the extensor mechanism and PCL. Thus, to obtain more flexion you should recess tight PCLs if you choose PCR type prostheses. Since the appropriate amount of PCL recession is not always easy, PCS type prostheses generally yield better flexion. To reduce tension of the extensor mechanism you should resect more patella than usual but this may cause postoperative patellar fracture. Or you can deepen the patellar groove by prosthesis modification but we should remember that both of these techniques will cause loss of the extensor lever arm and power. All posterior overhanging bone should be knocked out after trial reduction of a femoral prosthesis. Slightly flexed positioning of the femoral component and posteriorly tilted positioning of the tibial component can provide better flexion although too much of this positioning causes postoperative extension block.
Regarding the prosthesis design, PCS type prostheses can provide more predictable postoperative knee flexion. Other alternatives are a femoral component with a smaller AP dimension and deep patello-femoral groove. However, both of these will cause weaker extensor power. Posterior lip of the tibial polyethylene decreases the contact pressure in knee flexion but will prevent posterior roll back of the femur and can cause impingement in deep knee flexion. In the normal knee, extreme internal rotation of the tibia occurs in deep knee flexion and this rotation cannot be achieved by a currently available knee design. Mobile bearing prostheses may be needed to achieve better kinematics.
Aggressive postoperative rehabilitation is advised to prevent postoperative contracture of the soft tissue. Finally, although getting deep knee flexion is needed it should be remembered that ensuring postoperative stability and long-term survivorship should always be the most important goal for successful TKR.
The abstracts were prepared by Mrs Dorothy L. Granchi, Course Coordinator. Correspondence should be addressed to her at PMB 295, 8000 Plaza Boulevard, Mentor, Ohio 44060, USA.
