Technical Aspects of Total Knee Arthroplasty After High Tibial Osteotomy
Conversion of HTO to TKA requires careful surgical planning. Some potential pitfalls are related to previous surgery and include hardware removal, joint exposure, dealing with tibial deformities from the previous osteotomy, and managing soft tissue imbalances. These aspects must be evaluated preoperatively, and their solutions must be clear before beginning surgery. Although some surgeons consider TKA after HTO to be no more technically demanding than routine TKA,14,27 the current authors believe that most surgeons find it more demanding.
Hardware Removal, Incision, and Joint Exposure. The first step is often hardware removal. Because of the risk of impingement with the tibial stem, the authors agree with those who suggest their removal regardless of their type (eg, staples or plates).13,28 However, there is no consensus on the timing of the removal; some authors suggest a 2-stage procedure,29 whereas others prefer a 1-stage procedure.28 The former has the advantage of a more predictable operative time. Moreover, it is possible to take samples for microbiology at the time of removal. However, 2-stage procedures increase anesthesia time, patient discomfort, and costs (2 hospitalizations and 2 procedures). In cases of bulky hardware, the necessity of detachment of the anterior tibialis muscle for closing-wedge HTOs or disruption of the MCL and pes anserinus tendons for opening-wedge HTOs may require slower rehabilitation. In such instances, as well as when infection is suspected, a 2-stage procedure is preferable.
When planning the skin incision, previous scars must be carefully considered because skin necrosis is a major issue in this surgery. Because blood vessels come from the lateral branches of the genicular arteries, the most lateral incision is preferred.30,31 When this is not possible and a new incision is required, it should be at least 6 cm away from older incisions, and incision convergence must be avoided. Arthrotomy depends at least partially on the skin incision. To decrease the risks of skin necrosis, wide subcutaneous dissection should not be performed. In case of hypocorrection after HTO and recurrence of varus, a medial arthrotomy is indicated. In cases of hypercorrection and valgus knee, a lateral approach is preferred.32 When the arthrotomy is performed on the opposite side of a previous osteotomy, this increases the risk of patellar necrosis and fracture.11 Due to scar tissue, patella baja, and proximal tibial malunion (more frequent in cases of closing-wedge HTO), accurate joint exposure can be difficult.9–12,29,33 Care should be taken to increase joint exposure and avoid patellar tendon damage or avulsion. Although cutting the femur first can make exposure easier, at times additional steps must be considered. Quadriceps snip and V-Y plasty are commonly adopted and were necessary in 23% of patients reported by Gill et al34 and 40% of patients reported by Toksvig-Larsen et al.12 Unfortunately, they weaken the extensor mechanism and may lead to even greater patella baja or extension lag.33 Tibial tubercle osteotomy is a more demanding option. It was necessary in 12% of patients reported by Badet et al.28 A precise surgical technique is mandatory to avoid potentially devastating complications. The tibial tubercle fragment must be at least 7 cm long, and, to avoid the risk of nonunion, it should be fixed with 2 bicortical horizontal screws.
Tibial Resection. After HTO, alterations of the tibial mechanical axis are possible. They can be located either within the soft tissue envelope for wear and deformity caused by arthritis or outside this envelope for tibial malunions (Figure 1). Malunions include angulation and translation of the metaphysis on the diaphysis and can often be corrected intra-articularly at the time of TKA. However, in severe deformities, this could lead to soft tissue unbalance, and additional osteotomies can be required to correct the angulation and displacement of the proximal fragment.
Anteroposterior radiograph of the proximal tibial metaphysis after high tibial osteotomy. Tibial malunions must be carefully evaluated before prosthetic implant because they influence tibial component positioning.
On the frontal plane, 2 scenarios are possible: (1) hypocorrection and varus recurrence or (2) hypercorrection and valgus deformity. In both of these scenarios, an additional problem is the absence of reliable anatomical landmarks for the height of tibial resection. This is the consequence of medial tibial plateau wear and lateral tibial plateau lowering after the initial osteotomy.11,13 Although joint line variations of up to 8 mm are tolerated,35 soft tissue unbalance could occur with larger variations. Joint line raising leads to low patella and collateral ligaments slackness, whereas its lowering causes patella alta and retinacula tightness. Neyret et al11 suggest a tibial resection 5 mm below the medial plateau in case of hypercorrection and 6 mm below the medial plateau in case of hypocorrection when using a 9-mm polyethylene insert. In case of hypercorrection and major tibial deformity, the risk of deep MCL release or cut at the time of tibial resection should be considered. The axis of tibial resection is strictly correlated to its height. This axis can be either perpendicular to tibial mechanical axis, as in primary implants, or adapted to the local anatomy. The latter option is extremely challenging and can result in lower-limb malalignment and soft tissue unbalance. Tibial resection perpendicular to the mechanical axis is more reliable but can cause impingement between the tibial stem (when used) and the lateral tibial cortex (Figure 2).9,11,13 This occurs in almost 15% of patients and is more frequent after closing-wedge HTO.28 Impingement can be avoided by implanting undersized tibial trays and leaving the lateral tibial plateau partially uncovered or positioning the tibial tray more medially, with the risk of overhang and MCL impingement. Custom-made prostheses, offset stems, or simultaneous osteotomies represent the only options in cases of severe deformity. Metaphyseal malunions are possible even in the sagittal plane, with anterior or posterior shifting of the proximal end of the tibia over the diaphysis. This may lead to impingement of the tibial stem with the anterior or posterior cortex. However, this is easily predictable and can be solved by slight anterior or posterior translation of the tibial tray or by using offset stems.
Postoperative anteroposterior radiograph of a stemmed total knee arthroplasty. In severe tibial malunion, the use of stemmed tibial trays can lead to impingement between the stem and lateral cortex.
Slope variations are encountered even more frequently (Figure 3), especially after closing-wedge HTO. Normally, the tibial slope ranges between 5° and 7°.36 An increase in the tibial slope causes anterior tibial translation, increase in knee flexion, and possible instability in flexion; a decrease in the slope leads to posterior tibial translation, posterior capsule contraction, and decrease of flexion. This must be carefully evaluated preoperatively with accurate clinical evaluation and imaging studies. Tibial slope correction is mandatory for a good prosthesis outcome. However, this correction is performed through intra-articular resections, although the deformity is extra-articular. This is dangerous because it can result in flexion instability in cases of thick resection. In these situations, Winsord et al13 proposed linking the femoral resection to the tibial resection using thicker polyethylene inserts to compensate for this flexion laxity, as well as considering an overresection of the distal femur to balance the knee in full extension. Malunions are extremely difficult to detect in the axial plane; however, because they can lead to patellofemoral maltracking and instability, they must be carefully evaluated.10,11 The gold standard is computed tomography scan assessment, although clinical examination may detect asymmetry with the unaffected side in external or internal rotation. The correction of tibial malunions by modifying the position of the tibial tray is dangerous because it affects both tibiofemoral and patellofemoral kinematics. For large malunions, derotation osteotomy can be proposed. However, in cases of tibial tubercle osteotomy at the time of joint exposure, the tibial tubercle can be fixed more medially to improve patellofemoral kinematics.
Lateral radiograph of the proximal tibial metaphysis after high tibial osteotomy. An increase in tibial slope must be corrected when implanting a total knee arthroplasty to avoid implant instability in flexion.
Soft Tissue Balance. After tibial resection, the medial and lateral sides may not be balanced. A tibial osteotomy produces an extra-articular deformity that influences soft tissue tension. At the time of prosthesis implantation, a tibial bone cut perpendicular to the mechanical axis leads to asymmetric resection with a thin side and a thick side. This produces the so-called resection laxity, which increases the pre-existing imbalance.
In cases of hypocorrection after prior osteotomy and varus recurrence, medial plateau wear and moderate tibial metaphyseal malunion (similar to bony deformity in a constitutional varus knee) are present. The tibial resection creates a trapezoid space, which is thicker on the lateral compartment. Sometimes, a medial release is necessary, but because the lateral structures are generally only slightly retracted, this release is minimal. In these situations, a posterior cruciate ligament (PCL)-retaining prosthesis can be implanted. In cases of more severe deformities, there are additional problems. The correction of a metaphyseal deformity of 10° through tibial resection leads to an asymmetry of 1.5 cm if the mediolateral tibial distance is approximately 10 cm.37 This requires extensive medial compartment release that increases the tibiofemoral space and may lead to medial instability. Unfortunately, the cutoff value for correctable deformities is not always predictable during preoperative evaluation, even if stress radiographs are extremely helpful. Neyret et al11 reported that angulations of 9° or more are difficult to correct through soft tissue release. Generally, the closer the extra-articular deformity to the joint, the greater its influence on knee kinematics. In such cases, PCL retention is challenging, and proper extension balance may require its sacrifice. Moreover, the role of the PCL is even more important in flexion, and its section leads to an increase of the flexion space of approximately 4 mm.38,39 However, combined PCL sacrifice and wide medial release often result in increased instability. More constrained implants are then required. Sometimes slight medial overtension or lateral laxity can be accepted to avoid even more constrained implants. For more severe deformities (beyond 10°–15°), complete release of the MCL is required to obtain a proper rectangular space. Because this leads to severe instability, it must be avoided, and simultaneous corrective osteotomies or hinged implants must be considered.
In cases of hypercorrection after HTO and valgus knee, tibial resection is thicker on the medial side, leading to medial compartment laxity. In such conditions, a medial parapatellar approach can be used. In cases of lateral structure retraction after closing-wedge HTO or medial collateral ligament insufficiency after opening-wedge HTO, this unbalance is even more severe. To improve joint exposure, a Keblish approach should be performed, as well as ileotibial tract release and/or tibial tubercle osteotomy. The fat pad should be preserved to facilitate joint closure at the end of the procedure. To compensate mediolateral unbalance, Krackow et al40 proposed MCL and PCL retensioning through their transposition. Nevertheless, the current authors are mindful of the effects of these transpositions on knee kinematics. Thus, they perform minimal tibial resection and compensate soft tissue unbalance through PCL release or sacrifice and pie crust of the lateral capsule.41 The popliteus tendon and posterolateral corner release should be carefully evaluated and their section gently performed only when soft tissue balance is unsatisfactory. For more severe deformities (beyond 10°), more constrained implants and tibial stems (to ensure uniform metaphyseal loads distribution) must be considered.14 However, the correction of the deformity through lateral release may be dangerous and cause peroneal nerve damage. In these cases, some laxity persists after the procedure. Thus, some authors suggest simultaneous corrective metaphyseal osteotomy.10,11,29–37 A medial closing-wedge HTO is preferred for its lower risk of peroneal nerve damage. If these HTOs are performed proximal to the tibial tubercle, metaphyseal necrosis can occur. To avoid it, Badet et al28 suggested performing a combined tibial tubercle osteotomy that allows for a more distal metaphyseal osteotomy and for a patella height correction.