Total knee arthroplasty (TKA) is one of the most successful operations performed by orthopaedists with an incidence that is rising with each passing year. In 1998, it was estimated that 259,000 TKAs were performed and that number increased to 441,000 in 2004.7 This represents an average yearly increase of 9.4%. Despite well-documented long-term results with TKA, there has been a current trend in the United States towards an increasing demand for partial (uni- and bi- compartmental) knee replacements. Riddle et al estimated an increase in unicompartmental knee arthroplasties (UKAs) from 6,570 to 44,990 during the eight year period between 1998 and 2005 an average yearly increase of 32.5%.7 Improved surgical techniques and implant designs are thought to be responsible for this surge in the popularity of UKA.
Unicompartmental Knee Arthroplasty (UKA)
UKA is a surgery that has fallen in and out of favor over the past 30 years of joint replacement history. Refinements in patient selection, improved long-term outcome data, surgical technique and implant designs have fueled the renewed interest in this procedure (Figure 1). In properly selected patients UKA is an attractive alternative to osteotomy or TKA for isolated medial or lateral arthritic changes. Favorable initial and long-term results have been found with UKA compared to high tibial osteotomy.2 Similarly Berger et al found 98% 10-year survivorship with revision or loosening as endpoints in 62 modular, cemented UKAs.1 Stringent selection criteria is important for the long-term success of a UKA and in the ideal patient would include: noninflammatory osteoarthritis with a mechanical axis deviation of less 5 degrees of valgus or 10 degrees of varus from neutral; intact cructiate ligaments; a range of motion greater than 90 degrees; a flexion contracture no greater than 10 degrees; and body weight less than 90kg.
A typical UKA involves resurfacing of the medial or lateral compartments with preservation of the patellofemoral compartments (Figure 2). Small-incision and minimally invasive options are available for UKA patients with several centers performing the surgery as an outpatient. The recovery rate for UKA has been reported to be three times faster than that of TKA.3 Blending rapid rehabilitation and peri-operative pain management protocols has led to excellent early functional outcomes for UKA patients. However, it is important to recognize that patellofemoral and lateral compartment disease progression is not uncommon during 
the second-decade after implantation. Conversion from UKA to TKA can be performed with relative ease and good success when disease progression does occur.
Patellofemoral Joint Arthroplasty (PFJ)
Patellofemoral arthroplasty is another form of partial knee replacement in which the patella and the underlying trochlea groove are resurfaced (Figure 3). In patients with osteoarthritis of the knee, up to 10% of men and 24% of women will have isolated patellofemoral compartment involvement.6 In such appropriately indicated patients, long-term success has been reported at 15 years follow-up.4 If patella maltracking is addressed prior to PFJ, then progression of tibiofemoral arthritis is most often the limiting factor and can lead to failure in approximately 25% of cases.5
The overall conservative nature of a PFJ (preservation of the anterior and posterior cruciate ligaments, medial and lateral menisci and tibiofemoral articulation) affords a rapid recovery in a properly indicated patient population. Modern implant designs and sound surgical technique allow this procedure to be an excellent intermediate stage for those young patients with isolated patellofemoral arthritic changes prior to TKA.5 Contraindications for PFJ include inflammatory arthritis, patella maltracking, chondrocalcinosis, and tibiofemoral disorders
Bi-compartmental Knee Arthroplasty
Until recently UKA, PFJ and TKA were the only monolithic option for treating degenerative joint disease of the knee. A new bi-compartmental knee arthroplasty has been introduced to treat the relatively common findings of medial and patellofemoral compartment arthritic changes. The Journey Deuce (Smith and Nephew, Memphis, TN) allows for resurfacing of the medial and patellofemoral compartments, leaving the lateral compartment and anterior cruciate ligament intact (Figure 4). By limiting the amount of bone and ligament resection, it is believed this new device will afford a more rapid, complete return to function. Previously two separate implants (a patellofemoral replacement and a UKA) were required to treat bi-compartmental degenerative joint disease, or more traditionally, a TKA was performed. Rolston et al reported that 73 out of 100 consecutive 
patients being seen for arthritic knee pain were noted to have well-preserved lateral compartments in the setting of advanced medial and patellofemoral degenerative changes, thus supporting the need for a bi-compartmental device.8
By preserving the anterior and posterior cruciate ligaments, normal kinematics and proprioception about the knee is maintained, allowing a return to function akin to that of UKA. Similarly, preservation of bone stock and landmarks should allow for a relatively simple revision to a TKA when necessary. It is believed that just as there has been a rise in UKA utilization recently, a similar trend will occur as surgeons gain more experience with bi-compartmental knee arthroplasty. In a recent report, Rolston et al reported excellent early (33 month follow-up) results using the Journey Deuce bi-compartmental arthroplasty in 95 patients.8 The authors found less blood loss than TKA, an average range of motion of 117 degrees, and a high level of early function and patient satisfaction. Contraindications for this procedure include lateral compartment arthritic changes, rheumatoid (inflammatory) arthritis, severe deformities, ACL deficient knees, and fixed flexion contractures.
Conclusion
Partial knee replacements are an excellent means for managing isolated degenerative joint disease of the knee. In appropriately indicated patients, a relatively fast and more complete recovery can anticipated than with TKA. International registries have shown UKAs represent 10-15% of knee arthroplasties each year. In the United States, this number has risen from 2.5% to 7.7% over eight years, 1998-2005. In the era of less invasive surgery and rapid rehabilitation protocols, it is likely we will continue to see a rise in the number of uni- and bi- compartmental replacements being performed annually.
1. Berger RA, Meneghini RM, Jacobs JJ, Sheinkop MB, Della Valle CJ, Rosenberg AG, Galante JO. Results of unicompartmental knee arthroplasty at a minimum of ten years of follow-up. J Bone Joint Surg Am. 2005;87(5):999-1006.
2. Broughton NS, Newman JH, Baily RA. Unicompartmental replacement and high tibial osteotomy for osteoarthritis of the knee. A comparative study after 5-10 years’ follow-up. J Bone Joint Surg Br. 1986;68(3):447-452.
3. Deshmukh RV, Scott RD. Unicompartmental Knee Arthroplasty: Long-term Results. Rosemont, IL: AAOS; 2006.
4. Kooijman HJ, Driessen AP, van Horn JR. Long-term results of patellofemoral arthroplasty. A report of 56 arthroplasties with 17 years of follow-up. J Bone Joint Surg Br. 2003;85(6):836-840.
5. Lonner JH. Patellofemoral arthroplasty. J Am Acad Orthop Surg. 2007;15(8):495-506.
6. McAlindon TE, Snow S, Cooper C, Dieppe PA. Radiographic patterns of osteoarthritis of the knee joint in the community: the importance of the patellofemoral joint. Ann Rheum Dis. 1992;51(7):844-849.
7. Riddle DL, Jiranek WA, McGlynn FJ. Yearly incidence of unicompartmental knee arthroplasty in the United States. J Arthroplasty. 2008;23(3):408-412.
8. Rolston L, Bresch J, Engh G, Franz A, Kreuzer S, Nadaud M, Puri L, Wood D. Bicompartmental knee arthroplasty: a bone-sparing, ligament-sparing, and minimally invasive alternative for active patients. Orthopedics. 2007;30(8 Suppl):70-73.
Figures:
Figure 1. High-flex uni-compartmental knee replacement. Courtesy of Zimmer (Warsaw, IN) – Not Shown
Figure 2. (A) Pre-operative AP radiograph with medial compartment arthritic changes. (B) Post-operative radiograph after a medial unicompartmental arthroplasty.
Figure 3. Patellofemoral arthroplasty component. Courtesy of Zimmer (Warsaw, IN) – Not Shown
Figure 4. (A) Picture of the Deuce™, bi-compartmental knee arthroplasty. (B) AP radiograph showing an implanted bi-compartmental knee arthroplasty. Courtesy of Smith and Nephew (Memphis, TN)
(Article courtesy of Midwest Orthopaedics at Rush)
