Unicompartmental Knee Arthroplasty
N.P. Kort, MD, PhD
Nanne P. Kort, Jos J. A. M. van Raay, Jim J. van Horn
Knee Surg Sports Traumatol Arthrosc
Received: 3 February 2006 / Accepted: 23 August 2006
Treatment of younger patients with medial unicompartmental disease of the
knee joint remains a challenging therapeutic dilemma.With the refinement of
implant design, fixation and the minimally invasive techniques employed with
unicompartmental knee replacement, indications have expanded to include its
use in young patients. A prospective cohort of 46 unicompartmental knee
procedures were performed with a 2-year minimum and 6-year maximum
followup, using the Oxford phase III unicompartmental knee arthroplasty, in
the younger patient group (age 60 or younger). We conclude that the
unicompartmental knee arthroplasty is an important option for the treatment of
medial compartment disease for patients 60 years or younger. Obesity can
cause technical difficulties, increased risk of complications and early failure of
Younger patients with unicompartmental degenerative knee joint diseases
present a challenging therapeutic dilemma. These younger patients ask
normally more from their prosthesis as they are more active, and increased
wear and loosening of any prosthesis is to be expected 23.
High tibial osteotomy is the alternative treatment for these patients with
medial compartmental disease 5. However, obtaining the ideal valgus
position of the knee postoperatively is technically difficult and chances of
postoperative complications are greater than after placing unicompartmental
prosthesis 25. Comparative retrospective research shows results of the medial
unicompartmental knee replacement to be better than those of the
osteotomy 4, 12, 29.
The advantages of unicompartmental knee arthroplasty compared with a
high tibial osteotomy include higher rates of initial success and less early
complications 11, 12. Total knee arthroplasty (TKA) following high tibial
osteotomy is associated with more problems related to surgical exposure and
more technical difficulties compared to a conversion of a unicompartmental
knee arthroplasty to a TKA 10. It should be stated although that both
procedures are technically demanding.
The Oxford unicompartmental knee replacement shows survival rates of
94-98% in 10 years 14, 26 with the right patient selection, surgical techniques
and implant design 13, 18. The introduction of minimally invasive techniques
in the phase III emphasises smaller skin and capsular incisions, limited
quadriceps disruption, decrease in morbidity and decreased rehabilitation
time 19, 21, 22. Minimally invasive unicompartmental knee arthroplasty as an
initial arthroplasty procedure alleviates pain, restores the alignment and
improves the function with minimal morbidity, without interfering with
future total knee arthroplasties 20. The unicompartmental knee arthroplasty is
being performed with increasing frequency for the younger patient with
osteoarthritis 7 with good results 17.
Recent studies suggest that the results of the unicompartmental knee
arthroplasty in the younger patient group compare well with the results of
TKA in the similar age groups 16, 27.
The purpose of this study is to evaluate the mid-term results of the Oxford
phase III unicompartmental knee replacement in patients, age sixty or
younger by independent surgeons.
A total of 46 unicompartmental knee arthroplasty procedures were
performed between December 1998 and October 2003, using the Oxford
phase III unicompartmental knee arthroplasty, in the young patient group
(age 60 or younger). Forty-six unicompartmental knee arthroplasty
procedures were performed on 43 patients. Three patients had undergone
bilateral surgery in separate sessions. All persons gave their informed consent
prior to their inclusion in this prospective study. The youngest patient had
secondary unicompartmental osteoarthritis due to osteochondritis dissecans.
All the other patients suffered from primary unicompartmental osteoarthritis.
A strict patient selection criterion was used in all cases 13.
In the younger patients group 55.6% had a body mass index (BMI) of
more than 30. The BMI equals a person's weight in kilograms divided by his
or hers height in meters squared and correlates well with total body fat 3.
The 46 medial compartment arthroplasties were performed with
minimally invasive technique and under tourniquet control.
The discharge criteria were that their immediate postoperative pain had
settled and the operated knee was able to flex to a minimum of 90° with no
The results were prospectively recorded by means of a Historical Record,
Operation Record, Knee Society Score (KSS), SF-36 questionnaire and the
WOMAC Score. Postoperative data were available for every patient.
For the KSS ratings of excellent (85-100 points) and good (71-84 points)
were considered to indicate success.
All patients had at least 2 years of follow-up with a maximum of 6 years.
All patients had a weight-bearing antero-posterior and lateral radiograph of
their replaced knee with each visit at the follow-up clinic. The radiographs
were examined for loosening or radiolucency around the femoral and tibial
components and for the presence of osteoarthritis changes in the
nonreplaced compartment graded according to Ahlback classification 1. The
criterion for imaging was that a radiolucent line less than 2 mm thick
indicates no increased risk of loosening of the prosthesis 28.
We performed 46 unicompartmental knee arthroplasties in the young adult
group. At review, there were two knees (two patients) revised to a TKA.
The group consisted of 43 patients; 14 men and 29 women (3 patients
had undergone bilateral surgery). A total of 26 procedures were performed
on the left side and 20 on the right. The mean age at the time of surgery was
56 years (43-60). The mean operation time was 85 min (range 50-190 min).
The operations were performed by four different surgeons.
All the wounds healed primarily. No infections were documented in this
In the 41 patients with no revision, the average KSS knee score improved
from preoperative 42.22 [standard deviation (SD) 24.25] to 90.52 (SD 12.22)
at the latest follow-up evaluation and the KSS function score from 60 (SD
16.41) to 89.41 (Std Dev 17.71). Hundred percent of the patients had a good
or excellent KSS. The WOMAC pain score improved from 44.44 (SD 15.46)
to 76.38 (SD 14.99); the WOMAC function score from 46.41 (SD 14.94) to
73.45 (SD 16.36) and the SF-36 questionnaire showed improvement in role
emotional, role physical, physical functioning and bodily pain. The median
total range of motion measured 120° (range 0°-120°) before surgery and
125° (range 0°-125°) at latest follow-up.
No femoral or tibial component showed radiological loosening. There
were radiolucent lines less than 2 mm thick around six tibial components.
One of the patients had minor osteoarthritis changes affecting the lateral
compartment, graded as Ahlback 1, without symptoms. Six patients with a 1-
2 mm overhang of the tibial component on the medial side were
asymptomatic (Fig. 1). Three femoral components showed a malalignment,
more than 10° of varus or valgus (Fig. 2).
A separate statistical analysis (t test) comparing the result of the obese
(BMI > 30) and de-nonobese (BMI < 30) patient showed only a significant
difference (P value of <0.05) in the extension at the last follow-up. The obese
patients group revealed less extension (mean of 2°) at the last follow-up.
The first technical failure occurred in a 53-year-old woman with a BMI of
33. She had clinical and radiological loosening of the tibial component and
malalignment of the femoral component. Fluoroscopic evaluation showed
restricted gliding of the mobile bearing due to the malalignment. The knee
was revised after 11 months to a TKA.
The second technical failure had besides the malalignment of the femoral
component also serious peroperative complications. This 44-year-old patient
with a BMI of 39 had a popliteal vascular injury and the compartment syndrome of the lower leg, which are rare complications of knee
arthroplasties. Decompression of all four compartments of the lower leg was
performed about 22 h after the initial operation through a double incision
technique and 2 weeks later an arterial bypass with autogenous vein was
performed with good results. There is still a slight neurological (n.peroneus)
impairment. Two and a half years after surgery, revision to a total knee
replacement was required for failure of the prosthesis because of
Both revisions to a TKA were straight forward, using the standard primary
modular components, without the need for bonegraft. The third patient with
the malalignment of the femoral component was a nonobese patient and had
no complaints and needed no revision.
This is the first reported series of the Oxford unicompartmental knee
replacement phase III in younger patients in which only a minimally invasive
technique without everting the patella is used. This study showed that
younger patients with unicompartmental degenerative knee joint diseases
have adequate medium term results with an Oxford phase III
unicompartmental knee replacement. This technique shortens the early
rehabilitation period significantly without affecting the long-term
survivorship of the prosthesis 15, 19. The results were dependent on the weight
of the patients, the two failures had a BMI > 32. We think that the failures
represent technical errors that can be avoided.
This study shows comparable clinical results and few more complications
as the designer's series of younger patients 17. One of the two technical
failures in our series showed clinical and radiological loosening of the tibial
component and malalignment of the femoral component. We are of the
opinion that the loosening of the tibial component is due to abnormal
mechanics in the knee, caused by malalignment of the femoral component.
The gliding movement of the meniscal bearing is restricted, causing
abnormal stress on the tibial component. Therefore, this was considered a
surgical error due to the limited exposure and the obesity.
In the case of the popliteal vascular injury and the compartment syndrome
of the lower leg the aetiology for the increase in intracompartmental pressure
was probably the venous tourniquet and positional matters during the surgery.
The prolonged operating time due to the obesity and manipulation for the
correct positioning of the femoral component made matters worse. The
arterial injury is probably caused by manipulation with extreme varus-and
valgus-stress, rotation and hyper flexion, which probably lead to an intimal
tear which subsequently clotted off as may be seen following a knee
dislocation. Due to the fact that the lesion was 12 cm above the knee we do
not suspect that it was traumatized directly.
We have a quite large group of obese (BMI > 30) patients in our study. We
found no difference in the activity levels measured with the SF-36
questionnaire (physical functioning and role physical). The comparable
results in our study between obese and nonobese patients are as other shortand
mid-term studies of the TKA 6, 24. There are studies that suggest that any
degree of obesity has a negative effect on the outcome of TKA 8, 9. To date
there are view studies concerning the outcome of unicompartmental knee
arthroplasties in obese patients 2, 27. It should be kept in mind that the
technical difficulties, an increased risk of complications and early failure is
associated with obesity as seen in the two revision cases in our series. The
minimally invasive mobile-bearing unicompartmental knee arthroplasty is
technically demanding and complications occur with placement of this
unicompartmental knee device. With this minimally invasive technique, the
visual field is restricted.
The overhang of six tibia plateaus may also be considered as a technical
failure. All cases are asymptomatic, but only further follow-up can provide
information with regard to the importance of this finding. Only one of the
patients had minor osteoarthritic changes affecting the lateral compartment
We conclude that the age of 60 years or younger does not seem to be a
contraindication for this procedure. Obesity can cause technical difficulties,
increased risk of complications and early failure of this prosthesis; a BMI of
33 or more is a contraindication for the mobile-bearing unicompartmental
knee arthroplasty. When the nonoperative treatment of unicompartmental
arthritis in the knee fails in young nonobese patients and the symptoms are
obvious, mobile-bearing unicompartmental knee described in our study
provides the patients and surgeons a potentially attractive treatment option.
1. Ahlback S (1968) Osteoarthrosis of the knee. A radiographic investigation. Acta
Radiol Diagn (Stockh) Suppl 277:7-72
2. Berend KR, Lombardi AV Jr et al (2005) Early failure of minimally invasive
unicompartmental knee arthroplasty is associated with obesity. Clin Orthop Relat
3. Bray GA (1987) Overweight is risking fate. Definition, classification, prevalence,
and risks. Ann N Y Acad Sci 499:14-28
4. Broughton NS, Newman JH et al (1986) 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 68(3):447-452
5. Brouwer RW, Jakma TS et al (2005) Osteotomy for treating knee osteoarthritis.
Cochrane Database Syst Rev(1): CD004019
6. Deshmukh RG, Hayes JH et al (2002) Does body weight influence outcome after
total knee arthroplasty? A 1-year analysis. J Arthroplasty 17(3):315-319
7. Deshmukh RV, Scott RD (2002) Unicompartmental knee arthroplasty for younger
patients: an alternative view. Clin Orthop Relat Res(404):108-112
8. Foran JR, Mont MA et al (2004) The outcome of total knee arthroplasty in obese
patients. J Bone Joint Surg Am 86A(8):1609-1615
9. Foran JR, Mont MA et al (2004) Total knee arthroplasty in obese patients: a
comparison with a matched control group. J Arthroplasty 19(7):817-824
10. Hanssen AD, Stuart MJ et al (2001) Surgical options for the middle-aged patient with
osteoarthritis of the knee joint. Instr Course Lect 50:499-511
11. Ivarsson I, Gillquist J (1991) Rehabilitation after high tibial osteotomy and
unicompartmental arthroplasty. A comparative study. Clin Orthop Relat Res
12. Jackson RW (1998) Surgical treatment. Osteotomy and unicompartmental
arthroplasty. Am J Knee Surg 11(1):55-57
13. Murray DW (2005) Mobile bearing unicompartmental knee replacement.
14. Murray DW, Goodfellow JW et al (1998) The Oxford medial unicompartmental
arthroplasty: a ten-year survival study. J Bone Joint Surg Br 80(6):983-989
15. Pandit H, Jenkins C et al (2006) The Oxford medial unicompartmental knee
replacement using a minimally-invasive approach. J Bone Joint Surg Br 88(1):54-60
16. Pennington DW, Swienckowski JJ et al (2003) Unicompartmental knee arthroplasty
in patients sixty years of age or younger. J Bone Joint Surg Am 85-A(10):1968-1973
17. Price AJ, Dodd CA et al (2005) Oxford medial unicompartmental knee arthroplasty in
patients younger and older than 60 years of age. J Bone Joint Surg Br 87(11):1488-1492
18. Price AJ, Waite JC et al (2005) Long-term clinical results of the medial Oxford
unicompartmental knee arthroplasty. Clin Orthop Relat Res(435):171-180
19. Price AJ, Webb J et al (2001) Rapid recovery after oxford unicompartmental
arthroplasty through a short incision. J Arthroplasty 16(8):970-976
20. Rees JL, Price AJ et al (2004) Minimally invasive Oxford unicompartmental knee
arthroplasty: functional results at 1 year and the effect of surgical inexperience. Knee
21. Repicci JA (2003) Mini-invasive knee unicompartmental arthroplasty: bone-sparing
technique. Surg Technol Int 11:282-286
22. Romanowski MR, Repicci JA (2002) Minimally invasive unicondylar arthroplasty:
eight-year follow-up. J Knee Surg 15(1):17-22
23. Schmalzried TP, Szuszczewicz ES et al (1998) Quantitative assessment of walking
activity after total hip or knee replacement. J Bone Joint Surg Am 80(1):54-59
24. Stickles B, Phillips L et al (2001) Defining the relationship between obesity and total
joint arthroplasty. Obes Res 9(3):219-1223
25. Stukenborg-Colsman C, Wirth CJ et al (2001) High tibial osteotomy versus
unicompartmental joint replacement in unicompartmental knee joint osteoarthritis:
7-10-year follow-up prospective randomised study. Knee 8(3):187-194
26. Svard UC, Price AJ (2001) Oxford medial unicompartmental knee arthroplasty. A
survival analysis of an independent series. J Bone Joint Surg Br 83(2):191-194
27 Tabor OB Jr, Tabor OB et al (2005) Unicompartmental knee arthroplasty: long-term
success in middle-age and obese patients. J Surg Orthop Adv 14(2):59-63
28. Tibrewal SB, Grant KA et al (1984) The radiolucent line beneath the tibial
components of the Oxford meniscal knee. J Bone Joint Surg Br 66(4):523-528
29. Weale AE, Newman JH (1994). Unicompartmental arthroplasty and high tibial
osteotomy for osteoarthrosis of the knee. A comparative study with a 12-to 17-year
follow-up period. Clin Orthop Relat Res (302):134-137
Malalignment of femoral component
Example of tibial component overhang