Abstract
Hypothesis
The use of cartilage compensated virtual standing CT images for pre surgical planning improves the reliability of preoperative planning.
Materials and Method
Sampling included in this study were > 62 years of age (mean age 58.17 yrs ±3.54 yrs, range 55–62) with symptomatic isolated medial osteoarthritis, genu varum (mean varus 5.6°±2.6 °, range 2.1°–8.6°), good range of motion (flexion > 90° and flexion contracture < 10°) and with minimal ligamentous instability. All subjects had obtained a pre-op CT scan, MRI scan and weight-bearing long bone x ray. Post-op CT and long standing x-rays were taken prior to hospital discharge.
A virtual software suite (HTO-OP3D, Zapalign Inc, Seoul, Korea) was utilised to determine an optimal osteotomy site, hinge location and a gap necessary to achieve the targeted virtual passing point.
Prerequisite to performing the necessary calculations a virtual standing pose for each patient specific bone models was created using the following steps.
To transfer the pre surgical plan intra-operatively, a customised alignment jig was manufactured
Results
Analysing the data using three dimensional imagery the femorotibial angle was corrected from a mean varus 5.5° ±2.3 ° (range 2.1°–8.6°) to a mean valgus 4.1° ±1.1° (range 2.9°– 6.1°). There was no statistical difference [p = 0.514] between the virtual simulatedpre-op valgus vs actual post-op valgus results extrapolated from the post op CT data {−0.18° ±0.3° (range −0.7°–1.0°)}.
Analysing the same parameters using two dimensional standard X-rays, the femorotibial angle was corrected from a mean varus 6.6° ±2.9 ° (range 3.4°–10.6°) to a mean valgus 3.9° ±1.2° (range 2.9°–6.1°) respectively showing no statistical difference in average change in alignment measured using both modalities [p = 0.13].
The mean opening gap calculated using the three dimensional imagery vs two dimensional were 8.2mm ±2.9mm (range 5mm – 12mm) and 13.3mm ±3.3mm (range 10.2mm – 17.6mm) respectively, the difference between these data sets being statistically different [p = 0.03059]
The post operative evaluation of the posterior slope showed no statistical difference [p = 0.371] between the native slope {11.6° ±3.7° (range 5.3°–15.0°)} vs the post operative tibial slope {11.6° ±3.6° (range 6.2°–16.1°)} respectively indicating that the patients slope was well preserved.
Conclusion
The use of patient specific bone model images superimposed into a cartilage compensated full extension simulated weight bearing pose and used to calculate the femorotibial corrective alignment and opening gap produces predictable results that is not influenced by the condition of the patients soft tissue. Corrective angles and opening gap calculations using two dimensional weight bearing X-rays does not always relate to good surgical outcome, primarily due to the influence of laxity on the alignment in standing pose.
Furthermore, a patient specific clamping type surgical guide is effective to implement the pre surgical and aid in maintaining the tibial slope.
