Case ReportTwo cases of late medial instability of the knee due to hip disease after total knee arthroplasty

•There are few specific reports of late medial instability after TKA.•We described two cases of late medial instability of the knee due to hip disease after TKA, which required revision TKA.•Both cases required revision TKA with constrained knee prostheses due to the severe medial instability.•The late medial instability in current study was partially due to the same mechanism underlying the long-leg arthropathy and coxitis knee.•Constrained prostheses were applied for both patients, providing moderately good results.

IntroductionThere are few specific reports of late medial instability after total knee arthroplasty (TKA). We described two cases of late medial instability of the knee due to hip disease with osteoarthritis or rheumatoid arthritis after TKA, which required revision TKA.Presentation of casesAn 82-year-old woman experienced right femoral neck fracture due to a fall that required conservative treatment at age 77 years and underwent left TKA at age 80 years. A 68-year-old woman underwent left TKA at age 54 years, right TKA at age 64 years, and left THA at age 67 years. Both cases required revision TKA with constrained knee prostheses due to the severe medial instability. Hip-knee-ankle (HKA) angle, range of motion (ROM), Knee Society score (KSS) and functional score (FS) were evaluated pre- and postoperatively. Their respective HKA angle improved from 134° and 155° preoperatively to 184° and 179° postoperatively. KSS improved from −4 and 53 points to 59 and 100 points, respectively. FS improved from −10 and 58 points to 25 and 90 points, respectively. In the 82-year-old woman, ROM did not improve from −10-90° to −20-90°. On the other hand, in the 68-year-old woman, ROM improved from 0-110° to 0-125°. The late medial instability in the current case report was partly due to a similar mechanism underlying the long leg arthropathy and coxitis knee caused by hip joint degeneration.ConclusionsConstrained prostheses were applied for both patients, providing moderately good short-term results.