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Constrained implants are frequently used for primary total knee arthroplasty (TKA) in patients with moderate and severe genu-valgum (>10˚). This deformity presents corrective challenges for ligament release. The lateral-parapatellar approach has been advocated as an alternative to the traditional medial-parapatellar approach. Claimed advantages include better access for release of tight ligamentous structures, without requirement for release of the medial-collateral ligament.We present our comparative experience of the use of an unconstrained knee-replacement prosthesis inserted by the lateral-parapatellar approach in comparison to a constrained-knee prosthesis inserted via the medial-parapatellar approach.49 primary total knee-replacements in 48 (6 males, 42 females) patients were performed; 32 through a lateral-parapatellar approach (group L) using an unconstrained-prosthesis and 17 through a medial-parapatellar approach more often requiring a constrained-prosthesis (group M). Mean preoperative valgus angle was 18.5 (range 11-34˚). Patient demographics (p=0.7) and valgus correctability were similar between the two groups.There was no significant difference in the mean post-operative valgus angle. This was 4.2˚ (range 1-9.5˚) using the lateral-parapatellar approach and 5.3˚ (range 0.3-10˚), p=0.12, using the medial-parapatellar approach.Transient common peroneal injury occurred in 2 patients, both group L, in the presence of valgus angles of greater than 20˚. To date no joints have been revised, or are unstable.The use of a lateral-parapatellar approach, appropriate soft tissue release, and an unconstrained PCL-preserving implant, yielded in all cases a stable, well aligned knee arthroplasty. This represents a viable alternative to the constrained-prosthesis using a medial-parapatellar approach in patients with moderate and severe genu-valgum.