The Patella in Total Knee Arthroplasty

1. The Patella in Total Knee Arthroplasty

2. Introduction • Patella has long been regarded as a source of serious complications in TKA and the cause for reoperation • much attention was given to the surgical technique of patellar resurfacing as a means of reducing the complications • others have opted to avoid resurfacing the joint as a means of reducing the risk of complication

3. Whether or not to resurface the Patella in TKA • The patello-femoral articulation is frequently involved with arthrosis and is hypothesized to be a significant cause of pain • goal of resurfacing is to reduce the incidence of ant. knee pain. • Majority of surgeons resurface patella and acknowledge complications that can occur • however, others still choose to leave patella unsurfaced

4. Which pt. is the best candidate to forego resurfacing? • Patella with satisfactory articular cartilage and is free from crystalline disease • however, the cartilage articulating with metal usually deteriorates with time and late onset of knee pain. • Philosophy of those not resurfacing is that incidence of post op pain is less than incidence of post op complication in resurfaced joints.

5. Patellar Designs • Classis is the all-polyethylene component with 3 posts for cement fixation. • Articular geometry has included dome shaped surfaces, as well as those with a sombrero cross section • degrees of tilt tend to reduce the contact area in many of these designs • the dome shaped patella is usually the victim of cold-flow deformation on either side of its apex or wear that converts it to a more sombrero type shape

6. Shape of fem. comp. has recently received significant attention in location of patello-fem. articulation • Freeman first designed the roller in a trough prosthesis with a high incidence of pat-fem complications • this component featured absolutely no groove for the patellar articulation • currently, refinements in size,shape, and angulation of trochlear grooves made patellar comp less troublesome

7. Fixation • Uncemented patellar comp have failed in large numbers • to avoid cement fixation, metal backing required to provide a porous coating • this reduced the available thickness of polyethylene • these pat buttons that tracked lat., were exposed to large forces at the location where polyethylene was thinnest • soon metal skeleton exposed, metal-on-metal articulation produced synovitis and destruction of comp

8. Longevity of cemented all-polyethylene patellar buttons has been surprisingly good • 3 posts favored over single large central post to decrease stress-riser in the center of patella

9. Surgical exposure and its influence on Patellar Tracking • Conventional surgical approaches for TKA include the med. parapatellar approach and straight arthrotomy • these clearly disrupt some of the vasculature to the patella bone on the medial side • when checking tracking at completion of surgery, tendency to overestimate need for lat. patellar retinacular release • this may lead to patellar fx

10. Several different surgical approaches have been adopted in an attempt to dec. devascularization of the patella and enhance tracking • subvastus and midvastus arthrotomies preserve the supero-med. blood supply to the patella • no evidence that patellar tracking is superior with these approaches

11. Surgical Technique: Maltracking is the Root of all Patellar Problems • Hungerford published a report identifying relationship b/n malrotation of femoral or tibial components and patellar tracking pathology • established that complete revision TKA may be required for patellar maltracking • Virtually every patellar complication may have maltracking as its root • in pat. fx., patellar button is constrained in fem condyle and yet ext. mech. in patellar bone would like to dislocate lat, leading to fx.

12. Same mech. becomes resposible for patellar comp loosening. • Rhoads et al in mech testing that int. rotated fem. comp. impart larger forces to patella increasing wear. • Rotational malalignment difficult to assess radiographically • tib comp should be aligned with ext. mech. • Anything that int. rotates tib. comp., driving tubercle laterally, favors maltracking and disloc of patella

13. The fem comp may be difficult to place in the appropriately rotated position • several rotational landmarks identified but easiest is post articular fem condyles • most instrument systems reference from the post condyles • valgus knee often suffers from hypoplastic lat. fem. condyle both distally and post. • If used as landmark, will place fem. comp in internal rotation

14. Use the transepicondylar axis as a landmark • Resect patella parallel to ant. aspect of the patella • in addition, replicate the original patella thickness with new patellar construct • stiffness may result from thick patellar construct; fx. risked with very thin patella • remember articular facets are asymmetric • if button placed centrally, central ridge will be displaced lat., causing increased lat. retinacular tension and lat. subluxation or dislocation.

15. If a lat retinacular release is required in a knee that was tracking centrally before surgery, be concerned about a malpositioned fem or tibial comp • Patella that is tracking laterally on a merchant view prior to O.R. will likely require release intraop

16. Miscellaneous Patellar Conditions • Patella clunk syndrome • With post stabilized knees (esp IB II) • Piece of synovium on the quads tendon begins to catch on the ant leading edge of the femoral trochlea • Post stabilized components have an abbreviated trochlear articular surface to permit the knee to come into full extension without the tibial spine striking the ant aspect of the fem component • Synovium catches on this leading edge and becomes injured and hardens into a rubbery fibrous mass • This tissue creates the full patellar clunk syndrome • Pt will have above average flexion and on initiation of standing, the knee catches.

17. With further application of quadriceps force, an audible clunk accompanied by pain, and then able to stand fully. • Can also experience crepitation with bending under a load • Passive motion of the knee will not replicate these symptoms • Problem resolved with arthroscopic resection of fibrous tissue from the quads • Knee arthroplasty after a patellectomy can lead to a compromised result. • The patella, buttressing the extensor mech, often prevents post tibial dislocation • In the absence, surgeon should use a post stabilized prosthesis or techniques that ensure a retained PCL under satisfactory tension • Buechel described using a bone graft into the residual tendon to re-establish a bony fulcrum

18. The Patella at Revision Arthroplasty • Isolated patellar revisions may be considered for painful knee arthroplasty where patella was not resurfaced or for catastrophic wear or loosening of patellar component • If internal rotation of tibial or femoral component is identified, revision knee arthroplasty should be performed • Well fixed patella may be difficult to remove without damage to the residual bone • Use a high speed low torque burr with a diamond wheel metal cutting device

19. This is inserted b/n component and bone and easily cuts off the lugs from the button • Once prosthesis removed, the bone interface is exposed along with the resected lugs. • With revision arthroplasty, when the residual bone underneath a loose or removed patellar component is of adequate thickness, a new component can be applied • If inadequate bone, revision of prosthesis is ill-advised

20. Residual bone can be shaped to better accommodate the trochlear groove of the fem component • This will fxn much better than a patellectomy • Patellectomy should be avoided b/c it can lead to extensor mechanism strength loss and profoundly increase risk of rupture of extensor mech. • A gull wing osteotomy has proven effective if remaining bone is too thin for a new prosthesis

21. Microsagittal saw used to perform osteotomy from its sup to inf pole. • Performed with the patella everted, going from deep to superficial • Extensor mech along with the soft tissue is left intact • Medial and lat halves the open up like a gull wing • Apex of the osteotomy sits comfortably in the trochlear groove and the 2 halves conform to geometry of femoral prosthesis • May use bone graft to augment osteotomy

22. Ruptured Extensor Mechanism • Often regarded as worst complication in TKA, even more so than the infected knee • Simple repairs of the ruptured material is rarely successful • Extensor mech can be restored by several techniques • Engh described an autologous semitendinosis tendon graft in which the insertion of tendon is left in place and harvested tendon is looped up and over the patella

23. May also use an allograft • Consists of tibial tubercle, patellar tendon, patella bone, and quadriceps tendon • Trough is included in the pts own tibial tubercle and allograft tubercle is locked into this with screws or cerclage wire • May or may not resurface patella allograft • Quadriceps tendon is woven into host tendon with heavy nonabsorbable suture • Pts own residual extensor mech is closed over the top of the allograft • Needs to be implanted under considerable tension • Protect with hinge-brace

24. Conclusion • Current thinking favors resurfacing patella with a prosthesis to reduce amount of postop pain • Malrotation of components is probably the root of virtually every patellar complication • Careful attention to rotation will produce a centrally tracking patella with low risk for fx, wear, or loosening • In revision TKA, metal backed buttons should be revised and avoid patellectomy

25. Reshaping the residual bone can be very effective in restoring good fxn and comfort to the pt in the event of marginally inadequate bone stock • Dreaded complication of ruptured extensor mechanism responds very well to extensor mechanism allografts under considerable tension