Minimally Invasive Surgery

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CHAPTER 17 Minimally Invasive Surgery

Medial Fixed-Bearing Onlay Unicompartmental Knee Arthroplasty

William Macaulay, Amrit Goyal

Key Points

Select patients properly (carefully consider Kozinn and Scott criteria).

Avoid overcorrection (resect enough tibial bone and use a thin tibial insert).

Properly size and rotate both components.

Do not undersize the tibia: the tibial component spans the tibial spine and medial cortex of the tibia (no overhang).

Balance the flexion-extension gap.

Introduction

Unicompartmental knee arthroplasty (UKA) has gained popularity in the past decade. The concept of unicondylar knee replacement was introduced in the 1970s by Marmor. He reported pain relief in 86.6% of his patients at an average of 11 years of follow-up with this prosthesis.¹ However, Insall and Aglietti² and Laskin³ reported high failure rates of 26% and 20%, respectively, especially with medial compartment arthroplasty. Most of the failures in subsequent reviews have been attributed to inappropriate patient selection, technical errors, and poor implant design.¹ O’Rourke et al.⁴ recently reported a survivorship of 96% at 5 years and 84% at 20 years of follow-up with the Marmor prosthesis. Over the years, refinement in the surgical technique and improved implant designs and instrumentation have led to the resurgence of interest in UKA. The purported advantages of UKA over proximal tibial osteotomy are better pain relief, quicker return to function, fewer complications, and better long-term results. Furthermore, compared to a total knee arthroplasty (TKA), UKA has a more physiologic gait and better range of motion as the cruciate ligaments, contralateral menisci, and other structures are preserved.⁵ Also, the recovery is faster⁶ and it can be easily revised to a TKA with minimum morbidity if required for failure.

Since 2002, the senior author has been using a reproducible and accurate minimally invasive surgical technique (modified slightly from that described by Gesell and Tria ⁷), with a cemented modular fixed-bearing Miller-Galante (MG) implant (Zimmer, Warsaw, IN), which is described in detail in this chapter. Naudie et al.⁸ reported a 94% survivorship at 5 years and 90% at 10 years with the MG prosthesis. Argenson et al.⁹ also found survivorship of 97% at 5.5 years of follow-up with the same implant. Berger et al.^10,¹¹ reported Jorge Galante’s MG survivorship of 98% at 10 years and 95.7% at 13 years of follow-up. Minimally invasive surgery (MIS) has evolved over the years from more traditional UKA with the use of better instrumentation and modified surgical technique. MIS philosophy does not mean a small skin incision but aims at bone-sparing technique and preservation of all possible soft tissues. Also, during arthrotomy and surgery the quadriceps tendon is not incised and the patella is not averted, thereby preserving the integrity of the quadriceps mechanism.¹² MIS allows for a faster, less painful recovery, shorter hospital stay, and better cosmesis. Attempts to perform MIS surgery with a careless surgical technique can lead to component malposition, malalignment, and significant soft tissue damage. Coon¹² found that short-term results of MIS UKA were comparable to that of traditional UKA. Pandit et al.⁶ also found survival rates of 97.3% at 7 years with MIS technique. Since failures if they occur are more common early on, Pandit et al. believed that results of MIS should be comparable to the traditional UKA in the long term.

Preoperative Evaluation

One of the most important requirements for a successful outcome of UKA is patient selection. Kozinn and Scott ¹³ summarized their patient selection criteria in their UKA review paper from 1989. Those that we consider most important are as follows.

The ideal candidate for unicondylar knee arthroplasty is a low-demand, thin patient (>55 years of age) with isolated medial unicompartmental disease. The range of motion of the knee joint should be greater than 90° and a flexion contracture, if present, should be 5° or less. The valgus deformity should be less than 15°, and varus less than 10° should be passively correctable to neutral. There should be no rest pain or evidence of inflammatory arthropathy. There should not be diffuse pain or significant involvement of other knee compartments. The lateral compartment should have full-thickness joint space on the anteroposterior radiograph and also well-preserved meniscus and articular cartilage. Patellofemoral pain is a relative contraindication. The knee should be stable without any insufficiency of the cruciate or collateral ligaments. Medial or lateral subluxation or posterior tibial bone loss is suggestive of crucial ligament insufficiency. Patients with osteonecrosis of a single femoral condyle should not have extensive metaphyseal-diaphyseal involvement. Ideally the patient should be able to pinpoint the medial compartment as the source of pain, which worsens significantly with weight bearing (the so-called one-finger test).¹⁴

A thorough history, physical examination, and radiographic examination are done for all patients. Radiographic examination is of the utmost importance for medial UKA planning. The patient should have anteroposterior standing weight-bearing radiographs of the knee in extension and 45° of flexion (the so-called tunnel view) on a large film to judge the mechanical alignment. There should be no tibiofemoral subluxation or translocation, which denotes involvement of the contralateral compartment. A skyline view should be done to assess the patellofemoral compartment. The lateral view is helpful to determine the posterior tibial slope, look for any posterior femoral osteophyte that might impinge in deep flexion, and determine if the tibial wear pattern is excessively posteromedial (which could connote anterior cruciate ligament [ACL] insufficiency). A notch view or tunnel view (anteroposterior view with flexed knee) also helps in evaluation of cartilage thinning, which is much more impressive in flexion. Magnetic resonance imaging is required in some cases of osteonecrosis to evaluate the extent of involvement. It may also help in evaluating the other compartments’ and the cartilage thickness.

Surgical Technique for Medial Fixed-Bearing Onlay UKA

Positioning and Draping

We prefer that UKA patients undergo regional anesthesia unless contraindicated. The anesthesia team should be aware of the fact that the patient will be required to walk and start with the physical therapy soon after completion of surgery. Care is taken not to give drugs that might overly sedate patients for many hours. Patient preparation is done as per standard protocols. The patient is positioned supine on the operating table. The tourniquet is placed high up on the thigh so flexion contracture can be more easily seen, and an impervious drape is applied to prevent wetness below the tourniquet that could damage the skin. The patient operative site shaving is done just before prepping with a safety razor. A prescrub with chlorhexidine soap and water is done and the skin is prepped with tinted chlorhexidine solution. The skin of the extremity is draped with iodine-impregnated plastic draping to make the operative field impervious and decrease skin flora. Also, the MIS approach requires continuous repositioning of the knee to different degrees of flexion to optimize visualization, so care is taken to drape the extremity free with slackness in the drapes. We prefer a bump positioned at 90° of flexion, but a leg holder can be equally effective.

Approach

The skin incision (Fig. 17–1) is slightly medial to the midline, extending from the superior edge of the patella to the medial aspect of the tibial tubercle. The incision is approximately 10 cm long depending on patient size and adipose tissue, and can be extended to avoid applying too much tension to skin during retraction. A “mobile window” is created by releasing adipose tissue from the retinaculum approximately 2 cm circumferentially around the incision. A medial parapatellar arthrotomy (Fig. 17–2) is performed from a level 1 fingerbreadth above the superior pole of patella to a point approximately 15 mm distally to the tibial joint line. The arthrotomy does not damage the vastus medialis muscle. Also, the technique does not require eversion or dislocation of the patella, thus preserving the suprapatellar pouch. This may reduce postoperative pain and allow faster quadriceps control, since disruption of the extensor mechanism is minimal.

Figure 17–1 Skin incision.

Figure 17–2 Medial parapatellar arthrotomy.

The medial compartment is exposed by excising a small portion of the fat pad in the area just medial and posterior to the patellar tendon. All the lateral compartment structures and the cruciate ligaments are protected during the exposure. The patellofemoral compartment (easily visible), the lateral compartment (obstructed view), and the ACL are inspected. If significant arthritis is seen, conversion to TKA should be done, which should always be ready as a backup plan. The medial release is limited to a minimal medial subperiosteal release (enough to perform the tibial resection only). In varus knees that are not 100% correctable to neutral by a gentle valgus stress applied at 20° of flexion, the deep medial collateral ligament may be released if required to correct the alignment slightly (no more than 5°). Avoiding overcorrection of the knee alignment is essential to prevent loading of the opposite compartment, and therefore we prefer to undercorrect to about 2°. Any osteophytes from the intercondylar notch are removed to prevent impingement on the ACL.

Femoral Preparation

We use the intramedullary technique for femoral preparation. The site for inserting the intramedullary guide is 1 cm anterior to the origin of the posterior cruciate ligament and just anterior to the femoral intercondylar notch. With the knee flexed approximately 30°, a starting point is made with the punch. An 8-mm drill is used to make the path for the femoral intramedullary guide. The femoral canal is suctioned to remove the fat to decrease the chances of fat embolism. A fringe benefit of this hole is that it can be used for a patellar retractor later and for the entire case. The medial distal femoral resection guide (Fig. 17–3), which is attached to an intramedullary rod, is placed according to the side being operated. A shorter rod is used if the patient has altered femoral anatomy or a long-stemmed hip prosthesis. The guide is placed flush with the femoral condyle (which helps in making sure that proper thickness of bone is resected) and fixed in place if needed. The guide is stabilized with holding pins and the distal cutting adjustable valgus guide placed. The valgus angle is determined by evaluating the mechanical on preoperative standing radiographs. This usually corresponds to a 4° valgus cut on the distal femur. The distal femoral resection is deep enough only to match the depth of the femoral prosthesis. Medial osteophytes can be removed at this stage. Also, the surface may be rasped to remove any bony prominences to make it completely flat, which is important for subsequent jig placement.

Figure 17–3 The distal femoral intramedullary valgus cutting guide.

Next, the intramedullary patellar retractor (Fig. 17–4) is inserted (upon removal of the medial distal femoral resection guide), which allows for better exposure and prevention of patellar damage during the procedure. The femoral finishing guide (Fig. 17–5) is applied next to the distal femoral cut surface. Sizing and correct placement of this guide is one of the most crucial steps of the surgery. The size of the femoral component should be such that it leaves 1–2 mm of exposed subchondral bone on the anteromedial edge of the cut surface. The implant should be big enough to cover almost all of the condyle, but small enough to prevent patellar impingement. The guide rests on the posterior condyle and should be flush against the bone, or the accuracy of the cuts and subsequent component placement may be compromised. The femoral guide, properly rotated, is stabilized with holding pins and the anterior and posterior femoral posts are drilled and then chamfer and posterior condylar cuts made. An anterior femoral holding peg may be used to stabilize the guide while drilling the posterior post. After the cuts are made, posterior osteophytes are removed with a curved osteotome (Fig. 17–6) and curette to prevent impingement in deep flexion. The surfaces must be smoothed out of any bony irregularities. The femoral guide should be placed parallel to the long axis of the tibia at 90° of flexion and full extension. It is important to align the femoral component so as to later avoid edge loading on the polyethylene insert. The femoral component is generally smaller than the femoral condyle and, if in between sizes, the smaller of the two should be chosen. Attention is now paid to the tibial side.

Figure 17–4 Intramedullary patellar retractor is a key retractor for exposure.

Figure 17–5 Sizing and correct placement of the femoral finishing guide.

Figure 17–6 Curved osteotome used to remove any posterior femoral osteophytes.

Tibial Preparation

The tibial preparation is done using an extramedullary guide. Intramedullary instrumentation requires patellar eversion and wider exposure, which is difficult with MIS technique. The distal portion of the tibial alignment rod is positioned at the center of the ankle mortise (over the anterior tibial tendon) and held with the spring. The longitudinal axis of the rod is aligned parallel to the long axis of the tibia. Proximally the guide is fixed with the rod centered at the tibial spine. Care should be taken while draping to avoid heavy drapes around the ankle to help in alignment for the tibial cut. The posterior slope of the tibial cut is set to match the patient’s posterior tibial slope.¹⁵ A 2-mm depth gauge is used to take the tibial cut from the deepest defect in the tibia. By applying it a little loose (with regard to knob tightness), a 3- or 4-mm resection can be taken to avoid overstuffing the medial compartment. The tibial resection guide (Fig. 17–7) is fixed with guide pins and all of the knobs are tightened once the position has been confirmed. A retractor is placed medially to protect the medial collateral ligament and another in the intercondylar notch to protect the lateral structures. The front-to-back conservative vertical cut into the tibial spine is made with a reciprocating saw (preserving the attachment of the ACL) on the tibial surface. At the same time the cut is made as close to the ACL attachment as possible to get a broad tibial cortical support for the tibial baseplate. Also, care is taken to make the cut in proper rotation without damaging the posterior cruciate ligament. It is also critically important to avoid any anterior or posterior tibial bony cortical extension of the vertical cut and to avoid undercutting of the tibial spine.

Figure 17–7 MIS tibial cutting jig.

After the cut, the tibial resected surface (shaped like a “half-lollipop”) is removed (Fig. 17–8). Any irregularities of the tibia are smoothened with a rasp (Fig. 17–9). The osteophytes tenting the medial collateral ligament are removed, and removal of the medial meniscus may be easiest at this stage. The tibia sizer is used to determine the best tibial size for covering the tibial surface in both anteroposterior and mediolateral dimensions (Fig. 17–10). Also, the resected tibial surface can be used for estimating the size of the tibial baseplate. The largest possible tibial size is chosen, making sure that there is minimal to no overhang and the implant rests on strong cortical bone. An undersized tibial component has higher chances of subsidence, especially if the tibial resection is deeper than anticipated. A quarter-inch osteotome is used to make a starting slot for the central fin of the tibial baseplate (Fig. 17–11). The trial tray is then impacted into place and secured. The tibial post holes are prepared with the tibial peg drill to complete the tibial preparation.