Minimally Invasive Approach
The concept behind unicompartmental knee arthroplasty (UKA) is the replacement of only the damaged part of the knee and the preservation of as much normal tissue and bone as possible, to allow the restoration of normal kinematics.
In the past, UKA was performed through a standard anterior approach with dislocation of the patella, as was used for total knee arthroplasty (TKA). However, the minimally invasive form of unicompartmental arthroplasty has gained popularity. The smaller incision and arthrotomy of this technique cause less damage to the extensor mechanism, because the patella is not dislocated and the suprapatellar synovial pouch remains intact. (See the images below.)
Unicompartmental knee arthroplasty. The minimally invasive technique with a paramedial skin incision.
Follow-up research demonstrated decreased morbidity, faster rehabilitation, and reduced length of hospital stay among patients who undergo the less invasive medial UKA. With appropriate pain control, the surgery can safely be done as an outpatient procedure, providing substantial cost savings, and patients have reported feeling less pain postoperatively than they did preoperatively. Nevertheless, surgeons utilizing the minimally invasive approach must contend with a restricted visual field, which makes UKA a demanding procedure.
The minimally invasive technique can also be used for lateral UKA, although some authors prefer the standard total knee approach. With the less invasive technique, the exposure is limited, and thus, positioning the prosthetic components is more difficult.
Optimal postoperative alignment
The optimal tibiofemoral alignment after UKA has yet to be determined. Extremes of overcorrection and undercorrection are undesirable. Overcorrection may result in medial or lateral subluxation and increased loading of the unreplaced compartment (medial or lateral). Undercorrection causes varus or valgus alignment of the leg and can potentially overload the implant. [33, 34] Most surgeons advocate undercorrection of the mechanical axis by 2-3° to avoid overloading the normal compartment.
In a restrospective study of two matched groups of patients, Herry et al concluded that restitution of joint-line height in resurfacing UKA can be improved with robotic-assisted surgery. [35]
In general, medial and lateral soft-tissue releases are contraindicated, because knees requiring these have too much preoperative deformity for UKA. Angular correction is usually obtained with removal of peripheral osteophytes that tent the capsule and the collateral ligaments.
Computer-assisted surgery can improve the postoperative alignment of medial UKA over that of conventional techniques. Real-time information about the leg axis at each step of the operation should diminish the danger of overcorrection or undercorrection.
The navigation system may also be helpful in achieving a more precise component orientation. However, all investigations into the navigation system's use have consisted either of bone-saw or of cadaver studies. As a result of the limited exposure and restricted visual field in vivo associated with the minimally invasive procedure, it is difficult to apply the studies' results to the optimal positioning of components. Further development of the navigation technique will determine whether it is more accurate than the conventional method. [36, 37, 38]
Revision to Total Knee Arthroplasty
Converting a UKA to a TKA (see the images below) is somewhat more difficult than performing a primary TKA. The results are acceptable but arguably not as good as they are with primary TKA. Despite the benefit of the conservative bone cuts used for UKA, stemmed components and augments may be needed for bone loss associated with component removal or osteolysis.
Revision to a total knee arthroplasty; view after removal of the unicompartmental prosthesis.
Revision to a total knee arthroplasty. The earlier femoral-condylar bone cut/milling of the unicompartmental knee arthroplasty is usually less or the same as that for total knee replacement distal femoral (medial and lateral condyle) resection.
Revision to a total knee arthroplasty. The medial or lateral proximal tibial plateau resection of the unicompartmental knee arthroplasty is usually at the same level as the proximal tibial plateau resection for total knee arthroplasty.
Bone defects, if present, usually can be treated with a local autograft. The cumulative revision rate at 10 years is more than three times higher for patients in whom a failed UKA was revised to a further UKA than for those in whom it was revised to a TKA. [26]
Complications
Early
Infection, superficial and deep, is possible. With lateral UKA, palsy of the common peroneal nerve may occur, though this is more common with TKA in patients with severe flexion and valgus deformity.
A tibial plateau fracture (see the image below) may result if too much stress is applied with cementation of the tibial component. The vertical tibial cut may act as a stress riser. In general, forceful impaction of the component is minimal, and this complication should be unusual.
Unicompartmental knee arthroplasty. A tibial plateau fracture may occur when too much stress is applied with cementation of the tibial component or after adequate trauma. The vertical tibial cut may act as a stress line.
Ligamentous instability is rarely a problem in properly selected patients with an intact anterior cruciate ligament (ACL). Soft-tissue releases should be minimal. Knee joint stiffness may occur.
Late
Late infection may occur, usually from hematogenous seeding. In addition, prosthesis failure or loosening is possible.
The bearing of a mobile-bearing unicompartmental knee prosthesis may become dislocated, especially with lateral UKA (see the images below). A high proxima, varus tibial angle, along with damage to or overdistraction of the lateral soft-tissue structures, is thought to contribute to this problem. Polyethylene wear can occur, but it may be less with a mobile-bearing design.
Unicompartmental knee arthroplasty. When a mobile-bearing prosthesis is used, the bearing can become dislocated.
A systematic review by Ko et al found that the overall incidence of complications for mobile-bearing designs was not significantly different from that for fixed-bearing designs; however, mobile-bearing designs were more susceptible to reoperations in scenarios involving aseptic loosening, progression of arthritis, or implant dislocation. [14]
Undercorrection or overcorrection of the deformity and malpositioning of the components may cause late complications. In the case of overcorrection, excessive load on the opposite compartment might accelerate degenerative changes. Undercorrection places excessive load on the prosthesis, and loosening and failure may result. Improper placement of the components can cause subluxation of the tibia on the femur or impingement of the patella on the femoral component.
Most patients develop a radiolucent line in the tibial bone-cement interface. If these lines are less than 1 mm, they usually do not progress. The appearance of a similar line under the femoral component is expected but is not easily demonstrated because of the nonplanar form of the femoral bone-cement interface.
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Unicompartmental knee arthroplasty. The minimally invasive technique with a paramedial skin incision.
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Extension of the minimally invasive skin incision for a total knee arthroplasty.
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The mobile-bearing unicompartmental prosthesis.
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Unicompartmental knee arthroplasty. When a mobile-bearing prosthesis is used, the bearing can become dislocated.
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Revision to a total knee arthroplasty; view after removal of the unicompartmental prosthesis.
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Revision to a total knee arthroplasty. The earlier femoral-condylar bone cut/milling of the unicompartmental knee arthroplasty is usually less or the same as that for total knee replacement distal femoral (medial and lateral condyle) resection.
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Revision to a total knee arthroplasty. The medial or lateral proximal tibial plateau resection of the unicompartmental knee arthroplasty is usually at the same level as the proximal tibial plateau resection for total knee arthroplasty.
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Unicompartmental knee arthroplasty. A tibial plateau fracture may occur when too much stress is applied with cementation of the tibial component or after adequate trauma. The vertical tibial cut may act as a stress line.
