Background: The constrained knee prosthesis has the basic indication in revision arthroplasty, but the latest literature reveals that it takes place also in primary total knee arthroplasty in cases of knee osteoarthritis associated with major deformities. Material and methods: Present study is based on the surgical treatment, using the constrained knee prosthesis in the primary total knee arthroplasty, during 2019-2021, of 28 patients with knee osteoarthritis associated with severe deformities in varus or valgus, in the Big Joint Replacement Department, Clinical Hospital of Traumatology and Orthopedics, Chisinau. Results: In this study, the following criteria were evaluated: the type of deformity – valgus (10 cases) and varus (18 cases); the degree of deformation – for varus knees was on average 300, and for valgus knee – 250; bone attrition – 11 cases with bone defects where it was necessary to use augmentations; affected side – in 19 cases the right knee was affected and 9 cases the left one; the women/men ratio was 4/1; the mean age of the patients was 67.5 years; average duration of the intervention – 140 minutes; in 5 cases a lateral para-patellar approach was performed, of which 2 cases with tibial tuberosity osteotomy; complications – 1 case with intra-operative periprosthetic fracture and 2 cases with superficial infections of the operated joints. Conclusions: Osteoarthritis of the knee progresses rapidly, leading to severe deformities, significant bone defects and joint instability, which are indications to use the constrained prosthesis in the primary total knee arthroplasty, long-term follow-up is necessary to obtain the last conclusion, but from this study the constrained knee prosthesis like primary implant for special indication had promising results.