Choice of Implants in Total Knee Arthroplasty

1. Choice of Implants in Total Knee Arthroplasty Vincent P. Cannestra Aaron G. Rosenberg Presented by Spencer F. Schuenman, D.O.

2. Introduction • Over 110,000 total knee arthroplasties are performed each year • Should the surgeon use a device that retains the posterior cruciate ligament or substituting for it? • Does the patient’s bone or soft tissues require a nonconstrained or constrained prosthesis? • What is the trade-off in using a prosthesis that incorporates more or less conformity?

3. Conformity • Congruity of the articulating surfaces and may be measured by comparing their radii of curvature. This refers to the way in which the concave poly articular insert conforms to the convex femoral component.

4. Constraint • The resistance of the articular surface configuration to motions, usually rotational or translational. This can be influenced by the patient’s bony anatomy and soft-tissue envelope, or by the component’s design. It is important to realize that conformity is not synonymous with constraint.

5. The area of contact at the articular surface is directly related to the implant’s degree of conformity or congruity. • With higher conformity, there is a greater surface area for contact stresses per unit area and, as a result higher contact stresses per unit area will be present at the articular surface. • This can lead to poly damage and wear including pitting, delamination, and deformation.

6. In addition, more conforming inserts will have less freedom for excursion during motion and, therefore, greater shear stresses may be delivered to fixation interfaces, which in theory could lead to an increased risk of loosening.

7. Kinematics • Defined as the study of the geometry of motion. • Therefore, both the degree of conformity and constraint designed into the articulation surfaces will impact both the kinematics of TKA and its wear and failure characteristics.

8. Reducing the degree of conformity between the articulating components may result in improved kinematics and motion in a TKA, however, this may come at the expense of constraint, whereby there is increased translational and rotational movement at the articular surface potentially leading to increased contact stresses, poly damage, and wear.

9. Contemporary Implant Designs • Four primary classes: • Those that retain the PCL • Those that substitute for the PCL • Those that substitute for one or more of the collateral ligaments • Those that can be inserted with either retention or sacrifice of the PCL

10. Posterior Cruciate-Ligament—Retaining Prostheses • An attempt to reduce the amount of stress seen at the interface, implants with minimal constraint and less conformity have been developed. This can be accomplished with retention of the PCL.

11. Potential Advantages • Decreased conformity of the articular surface with PCL retention allows for the development of tension in the PCL during flexion which guides femoral rollback. • Femoral rollback occurs when the femur contacts an area more posterior on the tibia as the knee flexes. This permits both greater flexion and increases the effective lever arm of the quadriceps mechinism. • The claim of more normal kinematics and motion should be associated with less load and shear stresses.

12. Advantages (cont) • It is claimed that PCL retention accounts for improved proprioception as well as more anatomic kinematics at the tibiofemoral and patellofemoral articulations. • Other benefits include technically easier restoration or maintenance of the joint line during surgery. The joint line should be restored to within 3 mm of its normal location to allow normal kinematics • Retention also provides stability against posterior subluxation and dislocation.

13. Disadvantages of Posterior-Cruciate—Ligament retaining Implants. • This increases the difficulty of soft-tissue balancing. One must tension the ligament in both flexion and extension. • If the joint line is not restored to its anatomic or near normal level, higher contact stresses may develop in the poly, thus potentially increasing its wear.

14. Disadvantages (cont.) • If an excessively tight PCL develops due to a flexion-extension gap mismatch, it will cause a disproportionate amount of femoral rollback. The knee will then be too tight in flexion, and a painful knee with limited motion may result. • To compensate, some surgeons may opt to partially release or recess a tight PCL to improve ligamentous balance. This can weaken the PCL and potentially cause failure.

15. Posterior Cruciate Ligament Substituting Prostheses • In an attempt to improve stability as well as avoid the high contact stresses seen in cruciate-retaining devices, designers increased the amount of conformity in their articular surface to reproduce or substitute for the function of the PCL. • The implants have a central poly tibial post that articulates within the notch of the femoral component housing, the so-called post and cam mechanism. • The design prevents posterior displacement of the tibia on the femur and femoral rollback is determined by the post and cam mechanism.

16. Advantages of the PCL substituting Prostheses • Excision of the PCL eliminates the need to balance the ligament, and makes it less difficult to obtain equal flexion and extension. • Thus, PCL substituting prostheses have been recommended for knees that have a significant flexion contracture and/or valgus deformity.

17. Advantages (cont.) • Also suggested for revision arthroplasty because the soft-tissues and PCL may be compromised. • It may also compensate for a slightly greater flexion gap and allow more flexibility in the placement of the joint line. • Faris et al found the average joint line elevation of 5.6 mm did not compromise clinical function. • Elevations up to 8-10mm appears to be relatively safe without detrimental effect on functional outcome and performance.

18. Advantages (cont.) • They are also recommended when there is insufficient bone stock for resurfacing the patella or in the presence of a previous patellectomy. • The implant is also suggested for similar reasons of extensor disruptions such as patellar tendon avulsions. • The design prevents posterior subluxation of the tibia on the femur.

19. Disadvantages • The increased conforming articular surfaces cuts down on stresses of the poly articular surfaces, but lead to increased stresses to the implant-bone or cement-bone interface that may lead to greater mechanical loosening. • The design also requires more bone resection in the intercondylar notch of the femur to seat the housing/cam mechanism.

20. Disadvantages (cont.) • Several authors have reported complications unique to the PCL—substituting prosthesis • Patellar clunk syndrome-during flexion a fibrous nodule catches or impinges against the anterior-superior aspect of the femoral component housing the area of the intercondylar notch causing pain or a palpable clunk. • Dislocations of the implants have reported by several authors. This occurs as the articular post disengages from the femoral intercondylar housing during flexion and external rotation.

21. Semiconstrained Prostheses • A semiconstrained device is used in the presence of collateral ligamentous insufficiency. The design is similar to the posterior cruciate substituting design, but involves the use of a taller, larger central tibial post that fits into a deeper box in the intercondylar region of the femoral component. • These implants provide a large degree of rotational and varus-valgus stability in addition to the posterior stability of the standard post in box mechanism.

22. As a result, the implant itself may bear more of the load normally taken by the ligamentous structures present in the less damaged knee. Thus, these implants have increased potential for loosening at the implant-cement interface and are usually placed with IM stems that help distribute these forces over a larger area.

23. Indications for Semiconstrained Prostheses • A fixed valgus deformity is the most common indication and is most often seen in the elderly poplulation. • In knees with significant flexion contractures, a persistent flexion/extension gap mismatch may result despite extensive posterior soft-tissue releases and removal of posterior osteophytes.

24. Indications (cont.) • Biplane and triplane deformities are usually seen in juvenile rheumatoid arthritis and hemophilia arthropathies. Soft tissue releases in this group may create ligamentous laxity in multiple planes. • Lastly, a stiff ankylosed knee in extension will require extensive soft-tissue dissection and release, which may result in a grossly unstable knee. • Iatrogenic injuries to the medial collateral or other ligaments may also be an indication for the semiconstrained prosthesis.

25. Fully Constrained Prostheses • These implants are only rarely used today and are not recommended for standard primary or revision TKAs. • In the past they have been associated with high rates of failure and complications including infection, loosening, breakage, and revision. • Rotating hinged devices have been developed to reduce the torsional stress at the fixation interface. These results are better than the older prostheses but are worse than less constraining systems.

26. Indications for Fully Constrained Prostheses • These are best suited for patients with severe loss of bone stock, highly unstable knees, low activity demands, a life expectancy of less than 5 years. • They may also be appropriate if there is complete absence of the medial collateral ligament and a severe valgus deformity. • They have also been proposed for complex knee reconstructions and limb salvage after tumor resection surgery.

27. Meniscal Bearing and Mobile Platform Prostheses.