The proposed study is a prospective collection of health related personal data (observational study). A total of approximately 10 subjects with traditional medial SIGMA® High Performance Partial Knee System and 10 ACL deficient subjects with medial SIGMA® High Performance Partial Knee System are analyzed during sitting onto a chair and getting up, deep knee bend, stance and swing phase of level walking, downhill walking (ramp descent) and stair descent at least 12 months post-operatively, in terms of 3D fluoroscopic assessed tibio-femoral motion as well as simultaneous skin marker assessed whole leg motion and ground reaction forces. All the video-fluoroscopic testing is performed at the Institute for Biomechanics at the ETH Zurich. The UKA subjects are recruited based on the inclusion and exclusion criteria.

The primary objective is to quantify and describe the three-dimensional kinematics for patients with a traditional medial SIGMA® High Performance Partial Knee System (functional ACL and tibial posterior slope matching the native bone) during these daily activities.

The secondary objective is to evaluate patients with the same medial UKA implant (SIGMA® High Performance Partial Knee System) in an ACL deficient knee, where the UKA was implanted at a 50% reduced tibial posterior slope relative to the native knee.

The knee arthroplasty for each patient was implanted more than one year ago as per inclusion criteria of this study with the sole purpose of standard clinical practice of the recruiting surgeons.

Inclusion criteria (group 1):

• medial SIGMA® High Performance Partial Knee System due to medial OA
• Intact/functional ACL
• BMI ≤ 32
• good clinical outcome, KOOS >70
• no or very low pain VAS < 2
• Follow-up at least one year post-op
• Standardized general health survey score (SF-12) within the normal range for people in their age group
• Age ≥ 18 years

Exclusion criteria (group 1):

• Actual significant problem on lower extremities
• Misaligned UKA
• Deficient / non-functional ACL (Lachman Test)
• Any other arthroplasty at the lower extremities
• Patient incapable to understand and sign informed consent
• Incapable of performing the motion tasks
• Pregnancy

Inclusion criteria (group 2):

• medial SIGMA® High Performance Partial Knee System due to medial OA
• Deficient / non-functional ACL
• Central to posterior wear of medial tibial plateau (pre-op MRI)
• 50% reduced tibial posterior slope after UKA (post-op radiograph)
• BMI ≤ 32
• good clinical outcome, KOOS >70
• no or very low pain VAS < 2
• Follow-up at least one year post-op
• Standardized general health survey score (SF-12) within the normal range for people in their age group
• Age ≥ 18 years

Exclusion criteria (group 2):

• Actual significant problem on lower extremities
• Misaligned UKA / non-reduced posterior tibial slope (post-op radiograph)
• Intact / functional ACL
• Any other arthroplasty at the lower extremities
• Patient incapable to understand and sign informed consent
• Incapable of performing the motion tasks
• Pregnancy

The clinical investigators of the participating clinics (PD Dr. med. Fabian von Knoch, Gelenkzentrum Zürich; and Dr. med. Stefan Preiss, Schulthess Klinik) will identify potential subjects based on clinical records. PD Dr. med. Fabian von Knoch will recruit patients he operated on at the Gelenkzentrum Zürich while Dr. med. Stefan Preiss will recruit subjects from his patient population at the Schulthess Klinik.