In September 2008, a 72-year-old white woman underwent an Oxford mobile-bearing unicompartmental knee arthroplasty (UKA) (Biomet, Inc., Swindon, U.K.) for anteromedial tibiofemoral osteoarthritis to her left knee ( Fig. 13.1 ). This functioned very well for 3 months until she tripped and fell onto the knee. After this incident, she complained of medial-sided knee pain. Serial radiographs showed steady collapse of the medial plateau ( Fig. 13.2 ).
She was admitted in October 2009 for revision of the UKA to a total knee arthroplasty (TKA). She had had coronary artery bypass graft surgery in the past. She was fit and well with an American Society of Anesthesiologists (ASA) grade 2 on assessment of fitness for anesthesia and surgery. The operation involved removal of the UKA with a resection in situ and conversion to a cemented, cruciate-retaining TKA with a stemmed tibia ( Fig. 13.3 ).
When seen for review in January 2010, she was so pleased with her left knee that she requested surgery on the right knee. She underwent a primary right TKA in March 2010. When seen in September 2010, her only complaint was that she could not kneel. The range of movement was 2, 2, and 100 degrees in the right knee and 0, 0, and 95 degrees in the left knee.
This chapter describes a technique for revising a unicompartmental knee arthroplasty (UKA) by resecting the components in situ and then applying a primary total knee arthroplasty (TKA) design.
There must be no excessive bone loss.
The femoral distal cutting block is applied obliquely, because the implant is in situ.
Femoral implant rotation is assessed using Whiteside’s line and the transepicondylar axis.
Anterior referencing is used because of previous resection of the posterior condyle.
The in situ resection is most appropriate when revision is undertaken for implant malposition and progression of arthritis to the other compartment.
Always send specimens for microbiologic and histologic analysis to exclude sepsis.
Although the tibial plate is protected by a stem, cruciate-sparing implants can be used in most cases. The posterior cruciate ligament (PCL) is almost invariably intact, and the anterior cruciate ligament (ACL) usually is.
Always have a revision kit available, including long stems and wedges.
The rate of sepsis for aseptic UKA revisions to TKA has been reported to be 3%.
The functional outcomes may not be as good as with a primary TKA.
Always have a definitive diagnosis for the cause of pain before revision is undertaken.
Revision of a unicompartmental knee arthroplasty (UKA) to a total knee arthroplasty (TKA) is considered easy, especially when compared with revision of a TKA to a TKA. This means that there is a lower threshold for revision, as amply demonstrated in various Joint Registries. However, caution is still advised, and revision still requires care. UKA can be an alternative to upper tibial osteotomy (UTO), and revision of a UKA to a TKA is easier than revision of a UTO. It is also recognized that femoral bone loss is more difficult to manage than tibial bone loss.
Controversy reigns regarding the precise indications for undertaking a primary UKA compared with a primary TKA. One extreme view states that the other compartments must have normal articular cartilage, the patient must weigh less than 80 kg (or have a body mass index <35), no crystal disease should be present, and the patient must be older than 70 years of age with a nonactive lifestyle. With these criteria, fewer than 5% of patients with symptomatic knee osteoarthritis are suitable candidates for UKA. The opposite view does not exclude patients with these limitations, and in this case the potential candidates for UKA can comprise as many as 60% of those who present. The other factor to consider is that up 20% of TKAs are painful at 1 year after implantation, whereas the figure for UKA is 10% or less.
There is a temptation to revise a painful UKA when one would not do so for a TKA and even if no abnormality can be found. It is unusual for the revision to abolish the pain in these circumstances. Collier and colleagues reported a single-surgeon series of 254 fixed-bearing UKAs using a variety of designs. They found five factors associated with revision: younger patient, thinner polyethylene component, longer polyethylene shelf life, tibial component in greater varus, and more varus postoperative mechanical axis. Failure was not associated with gender or weight (up to 123 kg).
In our unit’s experience of revising the Oxford UKA in eighty-nine patients, nine were revised leaving a UKA in situ, fifty-three were revised to a primary TKA, and twenty-seven required augments and stems.
Indications and Contraindications
UKAs tend to have a higher revision rate than TKAs in the first 2 years after implantation. Early failure is almost always related to a technical error such as implant malposition, overstuffing of the replaced compartment, dislocation of a mobile bearing, loosening or dislocation of the femoral component, loosening of the tibial component, subsidence of the tibial component, or medial plateau fracture. At about 3 to 5 years after implantation, progression of the arthritis to another compartment is the main reason for revision.
Infection is unusual. In fixed-bearing UKAs, polyethylene wear tends to occur beginning about 9 years after operation.
Contraindications to Revision
Unexplained pain is a contraindication, and the temptation to revise a painful UKA even though no cause can be found should be resisted. The results of such revisions are poor, and the patients tend to be even more disgruntled afterward.
The following equipment should be prepared:
Standard primary TKA instruments and implants
Availability of modular TKA system with stems and metallic augments
Availability of bone graft mill
Microbiology laboratory available for prompt specimen culture
Consider histopathology service available for frozen section
Anatomy and Approaches
In medial UKA revision, the old wound is opened and extended. The wound usually lies medial to the patellar ligament and is extended laterally, both distally and proximally, the latter to reach the midline. A standard medial parapatellar approach is then performed (subvastus and midvastus approaches can be undertaken if that is the surgeon’s preference). The approach is identical to that used for a primary TKA.
With a lateral UKA, it is usually possible to perform a standard medial approach with a standard incision, ignoring the previous lateral wound, assuming that a minimal access approach has been undertaken.
Examination and Imaging
Examination is performed with the patient under anesthesia to confirm that there are no significant fixed deformities.
Plain radiographic films with long views of the femoral and distal shafts in both anteroposterior and lateral planes allow assessment of alignment. The amount of bone loss (usually on the tibia) should be noted. A decision should be taken as to whether this can be resected away, whether an augment or bone graft is required, and whether an extended tibial stem is needed to protect the construct.
The limb is set up as for a primary TKA. Antibiotic prophylaxis should be deferred until culture specimens have been taken. The limb is exsanguinated, and a tourniquet is applied. The skin is prepared and draped in the standard manner.
Step 1. Exposure of the Implant
The incision is made, and the wound is deepened to enter the knee joint. Fluid and soft tissue should be sent for culture to the microbiology laboratory and to histopathology for routine staining. The tissue is mobilized off the tibial plateau around to the midpoint to retract the medial flap and expose the medial femoral condyle. The patella may be everted or the lateral flap retracted to expose the lateral femoral condyle ( Fig. 13.4 ). My preference is to evert the patella and excise the infrapatellar fat pad, especially if it is a lateral UKA revision. The anterior cruciate ligament (ACL) is then excised.