Several risk factors for occurrences of PJK have been reported in the literature including age more than 55 years, large abnormal preoperative sagittal parameters (high lumbar lordosis and long thoracic kyphosis), application of pedicle screws, thoracoplasty procedures, big curvature correction, posterior and combined anteroposterior spinal fusion (i.e., distraction of the posterior tension band and other posterior intervertebral elements), fusion involving the lower lumbar spine and sacrum, osteoporosis and high body mass index(7, 8). The common agreement on direct postoperative radiographic prognostic criteria for PJK has not yet established. Maruo et al(15) suggested that improved postoperative radiographic sagittal alignment parameters are an important method to mitigate the development of PJK after adult deformity correction.
In this retrospective study, we have used four formulae, to predict the development of PJK based on immediate post-operative radiological films. We found that among the four proposed formulae, the position of the sagittal apex of lumbar lordosis (formula 3) is the best immediate postoperative radiographic predictor of occurrences of PJK among the four hypothesized formulae. It has a great predictive value OR 4.5 (95% CI 1.09-18.9). This means that the occurrence of PJK is almost 5 times high when the apex of LL is not at its theoretical position compared to when it is at its theoretical position. The second good predictor parameter in our study for the occurrence of PJK was GSA (formula 2). The predictive value for GSA was OR= 2.5, (95% CI, 0.67-9.38). This means that when GSA is > 450 the occurrence of PJK is 2.5 times compared to when GSA is < 450. Even though formula 2 failed to reject the null hypothesis as the CI includes 1. The 3rd good predictor for occurrences of PJK was formula 4, when the sum positive (LL+TK) was compared with negative-sum (LL+TK) the predictive value was OR 1.12, (95% CI 0.29-4.39). but failed to reject the null hypothesis as the CI includes 1. The least predictive formula for occurrences of PJK among the four proposed formulae was formula 1. When the LL and TK were equal or nearly equal to their theoretical value the predictive ability for occurrences of PJK was OR 1.08 (95% CI 0.31.-3.72). The CI includes 1, it also failed to reject the null hypothesis.
Lafage et al(16) in their virtual model stated that realignment surgery for the adult spinal deformity (ASD) can be extremely successful by restoring a balanced sagittal alignment. Rose et al(17) defined ideal GSA as PI + LL +TK less than 450 and they found that the formula has a great sensitivity in forecasting the success of pedicle subtraction osteotomy (PSO) for two years. Yagi et al(2) in their retrospective review of 157 cases, they found that more than 80% of patient with non-ideal global sagittal alignment (GSA> 450) developed PJK, and the OR for these patients was 29. Sebaaly et al(18) in their retrospective study of 250 cases of adult idiopathic scoliosis from a multicentric database, found that GSA >450 was associated with the occurrences of PJK, with OR 1.7. in this study we also found that GSA> 450 is highly associated with occurrences of PJK, with OR 2.5. This might be explained by the reason that increased GSA results in the positive (SVA) and increases the stress for the proximal and distal junctions of instrumentation that lead to the development of PJK(19).
Roussouly et al(11) after studying the sagittal alignment of 160 healthy volunteers divided the spine into four types based on inflexion point. Type 1 and type 2 had low PI, type 3 and type 4 had high PI. They found that the lower arc of lordosis is a key determinant of global lordosis (lordosis tilt angle, location of the apex, and the number of lordotic vertebras). Both low SS and low PI are accompanied by flat and short lumbar lordosis while high SS and high PI are accompanied by long and curved lumbar lordosis. They further proposed that the spine is well balanced when lordosis apex is at L4 for low PI and L3 for high PI. In our study, the occurrences of PJK were very high when apexes of sagittal lordosis were not at their theoretical position compared to when they’re positioned at their theoretical position. These findings are similar to what has been reported in the literature by other scholars(18). This may be explained as when the postoperative apex is located high above its theoretical position kyphosis has less room to be constructed and increase the risk of occurrences of PJK and other spinal abnormalities(20).
In this study, we found that both formula 1 and formula 4 have low predictive ability for occurrences of PJK. this similar to what has been reported recently in the literature(18).
Despite meeting the criteria of all formula above yet occurrences of PJK were still there. and even some of the formulae failed to reject the null hypothesis. This might be the reason that risk factors for occurrences of PJK are many. some have been reported in the literature, including age, low bone mineral density, the use of pedicle screws, thoracoplasty, combined anteroposterior fusion surgery, and long fusion surgery(21-23).
The most important finding of this study is that the theoretical position of the sagittal apex of lumbar lordosis is a preventive factor for the development of PJK. This finding can guide the surgeon to decide the level of pedicle subtraction osteotomy (PSO) or a suitable place to put lordotic cages during spine deformity correction. Lafage et al(24) in a multicenter retrospective study of 70 cases of adult spine deformity who underwent PSO found no difference in sagittal curve correction between L3 and L4 PSO. But in this study, we suggest that PSO be done at L4 in low PI and at L3 in high PI patients.
This study has some weaknesses which are: is a retrospective study design with some data missing and some patients lost follow-up, for example, the data for bone mineral density and Oswestry disability index scores were not found. It also is a single-center study with a small sample size. But despite these limitations, the results from this study are similar to what has been reported by other scholars.