Summary: Lenke 1AL and 1AR have been shown to be two distinct curve patterns with different levels for their lowest instrumented vertebra. This difference places the two patterns at different risks for adding‐on with the 1AR group 2.3 times more likely to experience this problem. In the 1 AR group fusing patients short of the vertebra above the stable one increases the risk while in the 1 AL group younger age and skeletal immaturity place the patient at risk.
Introduction: Previous work (Miyanji et al.) has demonstrated two distinct Lenke 1A curve patterns based on the tilt of L4 (1AL and 1AR). The purpose of this study was to evaluate the incidence of distal “adding on” in these two Lenke 1A curves patterns with the hypothesis that 1AR curves have a higher incidence of adding on than 1AL curves.
Methods: A query of prospectively enrolled AIS cases identified 219 patients with surgically corrected Lenke 1A curves followed for >2 years. These patients were grouped based on the pre‐op direction of coronal L4 vertebral tilt:1AL (left) and 1AR (right). The incidence as well as clinical and radiographic risk factors for “adding‐on” were identified for each group. Addingon was strictly defined as an increase Cobb angle of at least 5° with distalization of the lower end vertebra, or a change in disc angulation below the lowest instrumented vertebra (LIV) of 5° or greater between the first erect and 2 year f/u xrays. Cases of progression due to implant failure/pseudo were excluded.
Results: Forty‐seven (21%) patients met the defined criteria for adding‐on. The average increase in Cobb was 10.4° compared with 4° degrees in the non adding‐on group. Lenke 1AR curves were 2.3 times more likely to experience adding‐on (37/144 Lenke 1AR, 10/75 Lenke 1AL). Of the 1AR curves, the patients who added‐on were fused an average 2.0 levels above the stable vertebra, versus an average of 1.3 levels for the patients who did not add‐on (p=0.001). In contrast, for 1AL curve patients, younger age (12.7 vs. 14.7 years old, p=0.002) and lower Risser grade (70% vs.14% Risser 0, p=0.004) were the factors more likely to be associated with adding‐on. The selected level of LIV was not a determinant of adding‐on in these curves.
Conclusion: In the 1AR curve pattern, there is a greater tendency to choose the LIV too short and in order to prevent adding‐on, we recommend fusing distally to 1 level above the stable vertebra. In 1AL curves the risk of adding on appears less, although the most skeletally immature patients deserve special consideration and may benefit from an additional distal level of fusion.