Background: Total knee arthroplasty (TKA) is the surgical treatment for osteoarthritis knee. Results of treatment are
measured by clinical outcome and by longevity of the implant. The coronal plane axis of the tibial implant and the mechanical axis alignment of the leg are factors affecting outcome of the surgery. Mechanical axis malalignment results in early loosening, limited function, and reduced survivorship.

Objective: To determine if good tibial alignment correlates with mechanical axis alignment and clinical outcome.

Materials and Methods: This was a retrospective cohort study of 62 TKA patients. All TKAs were done by a single surgeon. The surgical protocol included the use of computer-assisted surgery and antibiotic cementing of the posterior cruciate-retaining knee without resurfacing of the patella.
Preoperative, postoperative clinical outcome, WOMAC score, and KSS was record. One year postoperative x-rays (tibia implant alignment, mechanical axis alignment) and clinical outcomes (WOMAC score, KSS) were record. The patients were classified as good tibial alignment (Group A) if the tibia implant alignment was +2° of the tibia axis (neutral) and outlier tibial alignment (group B) if the tibia implant alignment was more than 92° or less than 88°. Clinical outcomes and the
mechanical axis of good tibial alignment and outlier tibial alignment patients were compared. The good tibial alignment group was subdivided into neutral mechanical alignment (Group A1) if the mechanical axis alignment was +3° and outlier mechanical alignment (group A2) if the mechanical
axis was less than 177° or more than 183°. WOMAC score and KSS were compared between the good tibial alignment and neutral mechanical alignment group (Group A1) and the good tibial alignment and outlier mechanical alignment group (Group A2).

Results: The good tibial alignment group had significantly better postoperative mechanical alignment than the outlier tibial alignment group (p = 0.003). There was no statistically
significant difference in WOMAC score (p = 0.18) between the groups, but the good tibial alignment group had significantly higher (better) KSS scores (p = 0.04). Between the good tibial alignment with neutral mechanical alignment group and the good tibial alignment with outlier mechanical alignment group, there was no statistically significant difference in clinical outcome with either WOMAC score or KSS.

Conclusion: Tibia implant alignment in the coronal plane is one factor that affects postoperative mechanical axis alignment. If the tibial implant alignment is within +2° of neutral (90°), then the mechanical axis will be better aligned, although clinical outcome may not be improved. Clinical
outcome was not correlated with tibial implant alignment within +2° of neutral with mechanical axis alignment deviation of less than 3° and tibial implant alignment within +2° of neutral but with alignment deviation greater than 3°.

Keywords: Total knee arthroplasty, Coronal tibial alignment, Mechanical axis alignment, Clinical outcome, Osteoarthritis knee