Total knee arthroplasty (TKA) provides good results even for severe knee osteoarthritis (OA) patients. However, patients often suffer from post-operative pain and have long rehabilitation periods. In recent years, utilization of unicompartmental knee arthroplasty (UKA) has increased in an effort to decrease pain and shorten recovery compared to TKA. Moreover, the long-term results of the UKA have improved. Many surgeons now wonder whether TKA or UKA is better for patients with isolated medial knee OA. In Japan, the government has public insurance system and patients are able to receive the joint replacement surgery inexpensively. This study was conducted to compare the cost the public insurance and the patients co-payment for TKA and UKA.

We investigated a series of thirty TKAs and fifteen UKAs performed in Fussa Hospital (Tokyo, Japan) from July 2012 to April 2013. Data from two TKAs' were excluded since the patients had comorbidities (asthma and severe DM) that extended their hospitalizations. Patients were discharged from the hospital if they were able to go up and down the stairs or were able to conduct routine activities of daily living.

Total payments the hospital received averaged $19600 (S.D. $1600) for a TKA and $15200 (S.D. $1300) for a UKA. Patients paied averaged of $690 (S.D. $370) for a TKA and $470 (S.D. $170) for a UKA (Figure 1). The surgical fee was $3769 for both TKA and UKA, and was uniform thoughout Japan. The implant price averaged $6200 (S.D. $300) for TKA and $3900 (S.D. $200) for UKA, where prices were also determined by the government. Hospitalization averaged 28 days (S.D. 7 days) for TKA and 21 days (S.D. 6 days) for UKA. For both TKA and UKA, the total cost and the number of days in hospital were highly correlated (R = 0.92 and R = 0.96, respectively). A linear cost model suggests the TKA cost was $210 times days of hospitalization plus $13100 and the UKA cost was $220 times days of hospitalization plus $10000. Patients' payments were not correlated to the days of hospitalization (R = 0.22 and R = 0.45, respectively).

TKA and UKA are performed all over the world now and the number of the surgeries increases each year. Althouth each country has a different insurance system, many countries face an increasing and problematic economic burden for both patients and insurance organization (either public or private company). This study showed UKA is less expensive than TKA by $4400, an advantage that might complement the traditional view that UKA is less invasive and often has fewer complications for treating isolating medical compartment OA. For Japanese system, patients pay relatively little out-of pocket despite long hospitalization, and length of stay has a direct and significant effect on total cost for TKA and UKA