Basic Science Issues in Primary Total Knee Replacement

Donald T. Reilly, MD. PhD

Introduction

At the recent State of the Art Update in Orthopaedics 2000 in Whistler, British Columbia, Kenneth A. Krakow, MD, moderated a group of presentations on the anatomy and biomechanics of total knee arthroplasty (TKA).

Anatomy and Biomechanics of the Normal Knee

Donald T. Reilly, MD, PhD,[1] explained the importance of incorporating normal knee kinematics into total knee designs. The historical anatomical orthogonal approach to knee kinematics induces the "J-curve," or the instant center of rotation of the knee. Brunet and colleagues[2] have demonstrated that analysis using the mechanical axis to orient the plane of rotation produces a signal point that defines the instant center of rotation as the epicondylar axis. This simplifies prosthetic design because a single radius can be used in the prosthesis for flexion and extension of the knee. This also simplifies manufacture and may eliminate mid-flexion instability. Designs based on the "J-curve" would be expected to produce better kinematics if placed in anatomical position rather than in the externally rotated femoral position. With a single axis of rotation kept more posterior, the momentum for the extension mechanism is enhanced and stair climbing and rising from a chair may be improved.

Biological Analysis of Retrieved Total Knee Arthroplasties
Thomas W. Bauer, MD, PhD,[3] discussed the benefits of analyzing retrieved TKAs. Retrieval analysis allows investigators to determine whether the design goals of the arthroplasty have been met in the clinical setting. Bauer first described levels of ingrowth in uncemented knees. Ingrowth has been shown in the past to occur in 10% to 29% of uncemented tibiae. Dr. Bauer presented retrieval data from a 59-year-old woman with rheumatoid arthritis who died subsequent to a well-functioning TKA. The analysis for ingrowth with both coronal and sagittal sections of the specimen showed less than 5% ingrowth of the entire surface area. This small amount of bony ingrowth implies that channels allow debris to contact the bone-prosthesis interface and cause osteolysis.
In retrieval studies, differentiation between wear and deformation is difficult. Bauer presented his own analysis of "backside wear" on 105 retrieved tibial inserts of multiple designs.[3] A total of 22 inserts showed evidence of backside wear, whereas many more showed evidence of deformation. However, creep cannot be considered as wear. Thirty-one inserts showed eccentric patterns of wear; these patterns were more common in retained typed replacements of the posterior cruciate ligament. Bauer pointed out that the retrievals were from revisions and not necessarily from well-functioning total knee replacements. The high backside wear was attributed to thin polyethylene, high articular surface wear, and length of time in vivo. These studies question the clinical significance of backside wear.
Does Contact Area Matter?
Traditionally, TKA designs have focused on increasing surface area of contact to decrease stresses at the articulation. Avram A. Edidin, PhD,[4] challenged the dictum that contact area is the all-important variable in TKA wear.
Retrieved degraded tibial components have the appearance of "case-hardened failure," in which the predominant failure mechanisms are spalling and galling. These failure mechanisms result more from degradation than from stress. To investigate these mechanisms, Edidin developed a small punch test that causes multiaxial loading similar to that which occurs in the polyethylene of TKAs. This test can retrieve small specimens throughout the thickness of a polyethylene insert and shows that degraded areas occur in the subsurface region where stresses (regardless of conformity) are highest. Sterilization of the material in inert atmospheres decreases degradation and enhances resistance to stresses. These experiments postulate that degradation rather than stress is the primary reason for delamination in TKA.


Osteotomy-Unicondylar Replacement

Peter M. Bonutti, MD,[5] outlined the complexities facing orthopedic surgeons choosing the best procedure for TKA in patients with unicompartmental osteoarthritis. Tibial osteotomy is favored for the younger, highly active patient with good range of motion and a normal patellar-femoral joint. Contraindications to this procedure include multiple compartment involvement and inflammatory disease. Unless a closing wedge procedure is prevented by ligament laxity, Bonutti favors this procedure over the opening wedge procedure because of the higher complication rate secondary to device failure and graft collapse in the open wedge procedure. Although osteotomy has poor long-term results, it can "buy time" for younger patients to perform more strenuous activities and postpone the need for arthroplasty.
The ideal patient for unicondylar replacement is an older, thinner patient with good range of motion and lower activity levels. This type of arthroplasty requires adequate function of all ligaments and minimal deformity to achieve the highest success rate. Disadvantages of the procedure include the small number of patients who fulfill this criteria and the resulting lack of experience with the procedure for orthopedic surgeons coupled its level of difficulty.
TKA remains the gold standard for elderly patients. This procedure yields the most reliable pain relief and longevity in patients with full compartment involvement, poor range of motion, ligament instability, and severe deformity.


References
1. Reilly DT. Anatomy and biomechanics of the normal knee. State of the Art Update in Orthopaedics 2000. Whistler, BC: February 12-16, 2000.
2. Brunet ME, Kester MA, Cook SD, Haddad RJ, Skinner HB. Determination of the transverse centre of rotation of the knee using CAT scans. Eng Med. 1986;15:143-7.
3. Bauer TW. Biological analysis of retrieved total knees. State of the Art Update in Orthopaedics 2000. Whistler, BC: February 12-16, 2000.
4. Edidin AA. Does contact area matter? State of the Art Update in Orthopaedics 2000. Whistler, BC: February 12-16, 2000.
5. Bonutti PM. Osteotomy-unicondylar replacement-total knee replacement. State of the Art Update in Orthopaedics 2000. Whistler, BC: February 12-16, 2000.