Master Techniques in Orthopedic Surgery Knee CH27


Mobile-Bearing Medial Unicompartmental Knee Arthroplasty

Casey M. deDeugd and Rafael J. Sierra

INDICATIONS/CONTRAINDICATIONS The advantages of mobile-bearing Oxford knee (OUKA) over traditional total knee arthroplasty and fixed-bearing unicompartmental knee arthroplasty (UKA) are that it optimizes the congruency of the femoral and tibial components throughout normal range of motion, more closely replicates the anatomic meniscus by enabling angular and translation motion, and minimizes tibial contact forces and stress transfer through two joint interfaces. In addition, similar to fixed-bearing UKA, it may be performed through a smaller incision, results in less blood loss, lower transfusion requirements, shorter hospital stay, and a decreased rehabilitation period. 1,2 The indications for medial unicompartmental arthroplasty using OUKA have been well described. The tibiofemoral degenerative changes should be limited to the medial aspect of the joint and any patellofemoral cartilage damage should be confined to the medial facet of the patella. As such, OUKA should not be performed in patients who have severe range of motion abnormalities, par- ticularly if flexion is limited to less than 100° or greater than 15° flexion contracture, those with severe patellofemoral arthritis, or when the degeneration involves the lateral facet of the patella. An intact anterior cruciate ligament (ACL) has historically been considered important for OUKA. 3 In addition, patients must have a varus deformity that is passively correctable. A fixed deform- ity indicates a structurally shortened medial cruciate ligament (MCL), typically occurs with an incompetent ACL and is a contraindication to OUKA. 3,4 . Ideally, the degenerative changes must have a characteristic wear pattern restricted to anteromedial arthritis. 4 The proposed mechanism for this characteristic wear is that the cruciate ligaments maintain the normal arc of motion of the medial femoral condyle in flexion and prevents structural shortening. 4 In summary, OUKA is indi- cated for anteromedial degenerative arthritis and contraindicated in patients with any ligamentous compromise, a fixed deformity, or evidence of tricompartmental arthritis. Age and obesity are not contraindications to OUKA. 5-7 Why Femur First? The femur-first technique described by Shakespeare 8 using OUKA remedies some of the initial com- plications and technical difficulties associated with OUKA. Preparing the femur first provides a guide for tibial resection to avoid overresecting the tibia. Placing the femoral component in the coronal plane helps accurately determine the tibial cut and enables an appropriate contact point between the femur and the tibia. In addition, it sets up the appropriate alignment for the sagittal cut on the tibia to ensure that femoral and tibial motion is coplanar. Finally, preparing the femur first in its native location ensures that the obliquity of the native joint line is not changed, maintaining the forces that are placed through the knee during gait. PREOPERATIVE PLANNING Physical examination and radiographic studies are the mainstays of preoperative decision making to determine whether a patient has anteromedial arthritis and is a candidate for OUKA. Patients must always have a correctable varus deformity, usually between 5° and 15°. Radiographic studies should



PART V Alternatives to Total Knee Arthroplasty include 3-ft weight-bearing anteroposterior hip-to-ankle, lateral, and sunrise patellar views as well as varus- and valgus-stress views. Lateral radiographs show the orientation of the medial condyle on the tibial plateau and allow evaluation of the competency of ACL and extent of posterior involvement. A varus-stress view confirms the presence of bone-on-bone arthritis, whereas a valgus-stress view ensures a correction of the deformity and maintenance of the lateral joint space. Templating of the femur is necessary to choose the size of the femoral component. TECHNIQUE The patient is placed supine on the operating table, with the operative extremity restrained in an appropriate leg holder to ensure that full flexion can be obtained after draping. The hip should be abducted to 30° with a thigh support. Ensure the knee can be flexed to 120° with thigh support not impinging on popliteal fossa. A thigh tourniquet is placed on the operative extremity and the appropriate intravenous antibiotics are given. The center of the femoral head should be marked at the midpoint between the pubic tubercle and the anterior superior iliac spine (ASIS). The authors prefer using a roll of tape that can be palpated beneath the drape throughout the case, as needed. We use a mixture of ropivacaine, toradol, and epinephrine for perioperative local analgesia in combination with either a spinal or a general anesthesia. Approach With the knee flexed to 90°, make a paramedial skin incision extending from the medial border of the patella to 3 cm distal to the joint line down to the tibial tubercle, making sure to stay medial to the patellar tendon. Remove some fat pad and inspect the condition of the ACL and the lateral com- partment. We perform a midvastus approach preserving the quadriceps tendon. A mark is made for the entry point of the intramedullary (IM) femur guide about 1 cm anteromedial and superior to the intercondylar notch. With the knee flexed to 45°, make a starting point using a 5-mm awl. Keep knee flexed and drill the entry point. Use rod pusher, to push IM rod into the femur until the rod pusher abuts the femur (Figure 27-1). The IM rod is placed as a guide for flexion of the femoral component. The femoral component would be flexed approximately 10° from the sagittal plane of the femoral shaft. This guide can simultaneously be used as a patellar retractor. Preparation of the Femur The author removes the osteophytes on the medial aspect of the medial femoral condyle and notch with a rongeur before marking the center of the femoral condyle. Then the center of the femoral guide is marked with a marking pen. Alternatively, one can use the technique of Shakespeare by measuring the distance from the lateral edge of the medial femoral condyle as 13, 14, and 15 mm for small, medium, and large components, respectively. This will ensure the meniscal bearing to be seated 2 mm medial from the medial aspect of the wall of the tibial tray (Figure 27-2). Choose the size of the femoral component depending on preoperative templating and assessment of the size of the native femur from medial to lateral. There should be no overhang The curved femoral drill guide that is decoupled from the femoral guide is used to position and drill the femoral component position in place. Because the drill guide will slide medially into the tibial defect, removing some cartilage from the lateral aspect of the medial tibial plateau will help in stabilizing the femoral guide into position (Figure 27-3). Link the femoral drill guide to the IM rod with the hinged elbow of the guide pointing laterally to ensure the appropriate position in flexion and extension planes (Figures 27-4 and 27-5). Ensure that the guide is seated on the bone, and make the drill holes that should coincide with the marked center of the condyle. Drill the anterior smaller 4-mm hole followed by the 6-mm drill hole (Figures 27-6 and 27-7). Placing the Guide of the Posterior Femur Place the saw guide for the posterior femur cut into the drill holes on the femoral condyle and tap it in gently (Figure 27-8). Complete the posterior femur cut using a 12-mm broad oscillat- ing saw blade. Make sure that all ligamentous structures, especially ACL and MCL, are well protected by retractors. The thickness of the femoral bone cut should be comparable to what will be replaced with the femoral component (Figure 27-9). The saw guide is then removed using the slap hammer.


27 Mobile-Bearing Medial Unicompartmental Knee Arthroplasty

FIGURE 27-1. After midvastus exposure, the intramedullary rod is placed as a guide for flexion of the femoral component. In addition, note that it can be used as a patellar retractor.

FIGURE 27-2. Marked center of the medial femoral condyle.

FIGURE 27-3. Removal of a small amount of tibial cartilage on the most lateral and posterior aspect of the tibia before placement of the femoral drill guide is helpful in preventing the guide translating medial into the tibial cartilage defect.

Milling of the Distal Femur To begin milling the distal femur, use the zero spigot by placing it into the 6-mm drill hole until the flange is down on bone. It is designed to remove enough bone for the femoral component to seat and this sets your zero position for additional distal femur resection. To get the circular mill in, it will be

FIGURE 27-4. Place the femoral drill guide onto the femur such that the 4- and 6-mm drill holes lie over the center line of the femoral condyle. Note that the handle of the drill guide should be parallel to the long axis of the tibia.

FIGURE 27-5. Link the femoral drill guide with the hinged elbow of the guide pointing laterally. Flex and extend the knee until the arms of the guide fall “into place” and the intramedullary guide and guide are linked.

FIGURE 27-6. Drill the anterior smaller 4-mm hole followed by the 6-mm drill hole.


PART V Alternatives to Total Knee Arthroplasty

FIGURE 27-7. Drill holes have been made.

FIGURE 27-8. Place the saw guide for the posterior femur cut into the drill holes on the femoral condyle and tap in gently.

necessary to flex the knee to 60° and retract soft tissues. Do not start milling until you are on bone. When ready, push mill in the direction of the femoral shaft. Remove the small rim of bone that was not captured by the spigot. Second and subsequent millings are performed after tibial resection. Put the femoral trial into position (Figure 27-10). Apply the spherical gauge around the femoral trial. Preparation of the Tibia Place the extramedullary tibial guide parallel to the long axis of the tibia in the coronal and sagittal planes (Figure 27-11). Use the G-clamp to fix the tibial guide with a zero resection block and pin the tibial guide in place with one nonheaded pin (Figure 27-12). Place the tibial guide so that the recess is fully seated against the medial aspect on the patellar tendon. We advise noting the depth of resection before completing this cut and advise to switch the zero resection guide to 2 + to avoid overresection. Remove the G-clamp and check the resection level with a C-guide. Use a reciprocat- ing saw to make the vertical tibial cut. Make sure to stay medial to the ACL and aim the blade in the direction of the head of the femur (using the mark placed between ASIS and pubic symphisis).

FIGURE 27-9. After completing the posterior femur cut, compare the size of the discarded fragment to the thickness of the posterior femur on the prosthesis.


27 Mobile-Bearing Medial Unicompartmental Knee Arthroplasty

FIGURE 27-10. Placement of the femoral trial.

FIGURE 27-11. Place the extramedullary tibial guide parallel to the long axis of the tibia (coronal and sagittal planes) and the tibial resection guide should be clamped with the G-clamp device to the spherical gauge placed around the femoral trial.

FIGURE 27-12. The G-clamp is used to clamp the tibial guide with a zero resection block. The tibial guide is pinned in place with a one-headed pin.

The femoral component placement dictates the position and direction of this cut, and the center of the femoral head is used as a secondary check. Avoid bringing the hand up with the saw and inad- vertently making a deep cut posteriorly. Before beginning the horizontal cut, ensure retractors are protecting the MCL from damage. Use a wide oscillating saw to make the cut. Complete the medial meniscectomy and leave a small cuff medially to avoid damage to the MCL. Compare the resected tibial bone to size the opposite side tibial component. The thickness of the tibia removed must be large enough to accommodate the 3-mm feeler gauge. If needed, remove extra bone using the 0 or 2-tibial guides. Whenever the feeler gauge is placed in the gap, make sure the retractors are removed to avoid misinterpreting that the gap is too tight. The 3-mm feeler gauge should be easily inserted and removed without much force. Assessment of Flexion/Extension Gaps Assess the flexion gap first using the feeler gauge as described. If more tibial resection is necessary, recut with the 0 shim. Repeat the check using the feeler gauge. A minimum of 3-mm flexion gap is required. It is necessary to remove the feeler gauge before moving the knee into extension because the extension gap is always smaller than the flexion gap and there is a risk to avulse the ligaments if the knee is extended with the feeler gauge in place. Establish the extension gap with the knee in approximately 20° of flexion rather than full extension because of posterior capsular tightening. This gap is always smaller, and therefore a thinner metal feeler gauge set is used. If the 1-mm gauge is unable to be inserted, then the gap is considered to be zero; but it could also represent a gap even smaller than zero. The spigots used for milling the distal femur are increased in 1-mm intervals and are numbered accordingly. The formula for calculating the amount of bone to be removed from the distal femur is as follows: Thickness of bone to be removed = Flexion gap (in millimeter) – Extension gap (in millimeters) Thus, if the flexion gap is 4 mm and the extension gap is 0 mm, then you will use the 4 spigot. Once the gap is balanced, make sure that the plastic feeler gauge can be easily inserted in flexion and extension with minimal force.


PART V Alternatives to Total Knee Arthroplasty

Completing Femoral Preparation Use the femoral trimming guide to remove anterior and posterior osteophytes using the drill and lightly tapping posterior with a mallet. Remove posterior osteophytes. Inspect by digital palpation to make sure they have been removed. Sizing the Tibial Component and Trial Insertion The tibial template is placed so that the posterior and medial margins are lined up with the posterior and medial edges of the tibial plateau. Use the tibial hook to ensure that there is adequate placement. Then the guide is pinned into place. The final cut is made in the slot on the tibial template for the keel on the inferior aspect of the tibial component. Inserting the Femoral and Tibial Trials The femoral and tibial trial components are then inserted using the impactor. On the basis of the previous gap balancing, a meniscal bearing of appropriate thickness is inserted. The knee should be ranged through the flexion–extension arc with varus- and valgus-stress testing at 20° of flexion. Cementing Components Before cementing, it is recommended that the femoral and tibial surfaces be roughened by placing multiple small drill holes on the cut surfaces (Figure 27-13). The cementing can be done in either one or two stages, the tibial component first and then the femoral component. The cement is spread on the surface of the tibia in a thin layer and the component is placed (Figures 27-14 and 27-15). The tibial impactor is used to seat the component by impacting from posterior to anterior. The femoral component is cemented next. The femoral trial and feeler gauge are removed. The central hole from the guide is filled with cement and cement is also applied to the inside of the femoral component. The femoral component is tapped in with a mallet (Figure 27-16). Excess cement is removed and the appropriately sized feeler gauge is inserted to provide adequate compression of the tibial component. It is important to keep the knee flexed to 45° and apply an axial load. Excess cement is removed from the edges of the implant. After all the cement has hardened, the joint should be thoroughly irrigated. The final me- niscal bearing is then snapped into place (Figure 27-17). A Davol drain may be placed before closure.

FIGURE 27-14. Cementing of both components is done in separate stages. The tibial component is cemented first using a thin layer of cement on the tibial bone.

FIGURE 27-13. The femoral and tibial surfaces are roughened using multiple small drill holes.

FIGURE 27-15. Tibial component is placed and impacted with the tibial impactor.


27 Mobile-Bearing Medial Unicompartmental Knee Arthroplasty

FIGURE 27-17. The final meniscal bearing is snapped into place.

FIGURE 27-16. The femoral component is cemented in place.

The arthrotomy and skin are closed. The author uses a running PDS Quill suture for the arthrotomy and the skin incision is closed with interrupted 2-0 monocryl, running 3-0 monocryl, and Prineo.

POSTOPERATIVE MANAGEMENT Today, this procedure is performed in an outpatient setting. We recommend weight bearing as tolerated from the first postoperative day. Patients generally are able to begin ambulating within 1 to 2 hours of surgery. Our protocol is to place intraoperative Davol drains to bulb suction and send patients to outpatient recovery. These drains are pulled after several hours. During their postoperative recovery period, patients are evaluated by our physical therapy colleagues and given instructions on passive range of motion.

PITFALLS TO FEMUR-FIRST TECHNIQUE Improper Placement of Femoral Guide

As with any knee arthroplasty, it is essential to perform the cuts on the femur with the appropriate coronal and sagittal balance. Accurate placement of the intramedullary femoral guide is essential to avoid varus–valgus malrotation of the components. The flexion–extension alignment is determined by ensuring the guide is placed in the center of the condyle perpendicular to the axis of intercondylar notch. Several studies reported early failure because of femoral component loosening, which may be attributed to malposition of the femoral component. 9-11 Mariani et al hypothesized that extension of the femoral components, in combination with increased knee flexion, may result in edge loading and eventual loosening of the femoral component. 12 This has been thought to be more prevalent in fixed-bearing designs because of increased constraint. It has been shown that in the OUKA design, malrotation of up to 10° in the femoral component can be tolerated without adverse effect or increased risk of component loosening. 13 In addition, if the medial to lateral placement of the femoral drill guide is incorrect, the femoral component will be rotationally unbalanced causing asymmetric wear of the polyethylene, which is another common cause of early failure. Inadequate Posterior Bone Removal If the femoral trial is not sitting on the distal bone, then the posterior cut was made with slope and there may have been inadequate removal of posterior femur. Replacement of the guide and assessment of the posterior condylar cut should remedy this problem.


PART V Alternatives to Total Knee Arthroplasty

Overresection of Tibia The original tibia cut must be carefully planned, because it is the main determinant of tibial slope and component thickness that will affect ligament balancing and alignment. An inadequate tibial resection can lead to overstuffing of the tibial component. In contrast, resecting too much tibia can cause weakness to the remaining tibial plateau and risk for fracture. 14,15 The level of resection of the tibia can be difficult to determine secondary to asymmetric wear in many patients undergoing OUKA. We recommend using the 2 + cutting guide to err on the side of caution with the tibial cut because more distal femur can be taken in subsequent steps to avoid overstuffing. COMPLICATIONS The complications of UKA are similar to those of total knee arthroplasty, including aseptic loosening, infection, periprosthetic fracture, soft-tissue pain, and polyethylene wear. However, the complication unique to OUKA is polyethylene bearing dislocation. 16-18 The complications associated with OUKA have been attributed to improper patient selection and inadequate surgeon experience. In a study where all patients had to meet strict operative criteria and a single surgeon with significant experience, the 10-year survival rate of OUKA was 98%. 16 RESULTS The overall survival of OUKA has continued to improve as the implants become more sophisticated and surgeon experience with this technique increases. Initially, reports from Swedish registry data for OUKA from Lewold et al showed overall survival at 6 years of 89%, and then Murray et al reported an overall survival at 10 years of 98%. 16 More recent series of 124 patients showed a 95% overall survival at 10 years 5 and in a large series of 564 knees between 91% and 96% at 10 years. 6 These data are comparable to results obtained from fixed-bearing UKA. 5 A known complication of OUKA or any mobile-bearing UKA is component dislocation, which is not seen in fixed-bearing UKA. This complication is rare, and it is avoided by appropriate surgical technique and rigid candidate selection criteria. The femur-first technique was initially proposed by Shakespeare et al to avoid problems typically associated with the OUKA, and their results showed adequate alignment of components and no evidence of bearing dislocations. 8 In summary, it is our opinion that in the appropriately selected candidates, the femur-first OUKA provides comparable long-term survival and complication rates as fixed-bearing UKA and OUKA via the standard technique. 1. Price AJ, Webb J, Topf H, Dodd CA, Goodfellow JW, Murray DW. Rapid recovery after oxford unicompartmental arthro- plasty through a short incision. J Arthroplasty . 2001;16(8):970. 2. Robertsson O, Borgquist L, Knutson K, Lewold S, Lidgren L. Use of unicompartmental instead of tricompartmental pros- theses for unicompartmental arthrosis in the knee is a cost-effective alternative. 15,437 primary tricompartmental prostheses were compared with 10,624 primary medial or lateral unicompartmental prostheses. Acta Orthop Scand . 1999;70(2):170. 3. Goodfellow J, O’Connor J. The anterior cruciate ligament in knee arthroplasty. A risk-factor with unconstrained meniscal prostheses. Clin Orthop Relat Res . 1992;(276):245. 4. White SH, Ludkowski PF, Goodfellow JW. Anteromedial osteoarthritis of the knee. J Bone Joint Surg Br . 1991;73(4):582. 5. Svard UC, Price AJ. Oxford medial unicompartmental knee arthroplasty. A survival analysis of an independent series. J Bone Joint Surg Br . 2001;83(2):191. 6. Price AJ, Dodd CA, Svard UG, Murray DW. Oxford medial unicompartmental knee arthroplasty in patients younger and older than 60 years of age. J Bone Joint Surg Br . 2005;87(11):1488. 7. Argenson JN, O’Connor JJ. Polyethylene wear in meniscal knee replacement. A one to nine-year retrieval analysis of the Oxford knee. J Bone Joint Surg Br . 1992;74(2):228. 8. Shakespeare D, Waite J. The Oxford Medial Partial Knee Replacement. The rationale for a femur first technique. Knee . 2012;19(6):927. 9. Skyrme AD, Mencia MM, Skinner PW. Early failure of the porous-coated anatomic cemented unicompartmental knee arthroplasty: a 5-to 9-year follow-up study. J Arthroplasty . 2002;17(2):201. 10. Bartley RE, Stulberg SD, Robb WJ III, Sweeney HJ. Polyethylene wear in unicompartmental knee arthroplasty. Clin Orthop Relat Res . 1994;(299):18. 11. Hamilton WG, Collier MB, Tarabee E, McAuley JP, Engh CA Jr, Engh GA. Incidence and reasons for reoperation after minimally invasive unicompartmental knee arthroplasty. J Arthroplasty . 2006;21(6 suppl 2):98. 12. Mariani EM, Bourne MH, Jackson RT, Jackson ST, Jones P. Early failure of unicompartmental knee arthroplasty. J Ar- throplasty . 2007;22(6 suppl 2):81. 13. Gulati A, Chau R, Simpson DJ, Dodd CA, Gill HS, Murray DW. Influence of component alignment on outcome for unicompartmental knee replacement. Knee . 2009;16(3):196. 14. Pandit H, Murray DW, Dodd CA, et al. Medial tibial plateau fracture and the Oxford unicompartmental knee. Orthopedics . 2007;30(5 suppl):28. REFERENCES


27 Mobile-Bearing Medial Unicompartmental Knee Arthroplasty

15. Sloper PJ, Hing CB, Donell ST, Glasgow MM. Intra-operative tibial plateau fracture during unicompartmental knee replacement: a case report. Knee . 2003;10(4):367. 16. Murray DW, Goodfellow JW, O’Connor JJ. The Oxford medial unicompartmental arthroplasty: a ten-year survival study. J Bone Joint Surg Br . 1998;80(6):983. 17. Marmor L. Unicompartmental knee arthroplasty. Ten-to 13-year follow-up study. Clin Orthop Relat Res . 1988;(226):14. 18. Ji JH, Park SE, Song IS, Kang H, Ha JY, Jeong JJ. Complications of medial unicompartmental knee arthroplasty. Clin Orthop Surg . 2014;6(4):365.

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