Incidence and mechanism of PJK after GR treatment
According to previous studies, the incidence of PJK in EOS patients after GR treatment varied from 12% to 56%, which was mainly due to different inclusion criteria and surgical strategies[10-12]. The incidence of PJK in our study cohort was 21.1% over an average 52.8±22.2 months of follow-up. It's remarkable that PJK in this study mainly occurs after the first distraction surgery. Li revealed that all patients developed PJK within the first year after VEPTR treatment. On the other hand, Wijdicks et al. reported that the occurrence of PJK in patients treated with magnetically controlled growing rod increased as time progresses but had no correlation to the number of distractions.
Several studies tried to explain the patho-mechanism of PJK. A biomechanical study on adult scoliosis suggested that the dissection of posterior intervertebral elements might lead to the increase of the mechanical stresses within the junctional area. Also, the injury of the paraspinal musculature and the extensive dissected facet joints during the surgery may destroy the integrity and stability of the posterior structures of the spine. According to our experience, the lengthening procedure itself is a local distraction surgery, which has a tendency to increase the kyphosis of the implant area or junctional area, eventually leading to the occurrence of PJK. In our study, the most common type of PJK was ligamentous failure which developed in 85% (17/20) patients. Our finding is in line with the study by Chen, but inconsistent with the study by Wang who reported that implant/bone interface failure was the major type of PJK in congenital scoliosis patients receiving posterior short fusion.
Association of PJA-PRCA mismatch with PJK
Currently, there are still no studies reporting the mismatch between PJA and PRCA as a risk factor for PJK in EOS patients receiving GR treatment. As a parameter to represent the proximal rod bending, the PRCA stays unchanged because the position of the screw and the rod is relatively fixed after the index surgery and allows us to indirectly measure the contouring of the proximal part of the rod. Thus the PJA-PRCA was proposed to reflect the matching between the proximal rod contouring and supradjacent regional spinal alignment. In our study, there was no significant difference in PRCA and preoperative PJA-PRCA between the two groups. However, the PJK group has a significantly higher proportion of patients with a value of PJA-PRCA greater than 5° than that in the non-PJK group. We believe that PJK might be avoided if the postoperative PJA-PRCA matches well. But when the proximal area of the instrumentation is excessively corrected, resulting in mismatched postoperative PJA-PRCA, the risk of PJK will be greatly increased. Of note, similar findings have been observed in previous studies. Yagi et al. reported that the inadequate restoration of global sagittal alignment was a significant risk factor for PJK in adult scoliosis patients undergoing long instrumented spinal fusion. Yan demonstrated that mismatch between postoperative PJA and PRCA led to a proximal compensation, namely PJK in degenerative scoliosis. After reviewing 84 adolescent idiopathic scoliosis patients, Wang et al. found that larger postoperative PJA-PRCA, especially those with PJA-PRCA greater than 5° were risk factors for PJK.
In the previous literature, two methods of contouring angle measurements of the proximal part of the rod were reported. In Yan’s study, PRCA was defined as the angle between the first two proximal screws in sagittal plane after surgery in DS. The second method is that of Lonner used in AIS patients and the PRCA was defined as the cobb angle between the UIV and once vertebra caudal to the UIV. These two methods have one thing in common, that is, they both use a contouring angle between 2 vertebra (UIV and UIV-1). In our study, we proposed a new measurement method to reflect the proximal rod contouring (PRCA was defined as the angle between the cephalad endplate of the UIV and the lower endplate of UIV-2).
Consequently, we propose some possible mechanisms. A large PJA-PRCA means the proximal rod is contoured insufficiently, resulting a greater biomechanical stress concentrated in the junctional area and leading to the occurrence of PJK subsequently. Moreover, to get satisfactory correction results, the proximal region may sustain excessive forces during the surgery. According to the Hueter-Volkman principle that compression forces inhibit growth and tensile forces stimulate growth, the posterior column of spine grows faster than the anterior column after the index surgery, further aggravating the kyphosis of the proximal junctional area. Therefore, even if the PJA is less than 10° before the lengthening procedures, it may progress to more than 10° after the subsequent distraction. Another cause of PJK is related to the particularity of GR treatment itself. GR treatment is different from other operations in that besides the index surgery, subsequent lengthening procedures are also needed. The stress generated during the lengthening procedures will be transferred to the junctional area, making the stress concentration, and finally lead to the occurrence of PJK. This is consistent with Bess et al.12 findings on the complications of GR treatment.
General risk factors of PJK
In this study, preoperative hyperkyphosis was also identified as an independent risk factor associated with PJK. This is also consistent with previous research. Chen et al. reported that hyperkyphotic EOS (TK≥50°) tended to experience increased complications, such as rod fracture and PJK. Lee et al. pointed out that correction surgery would impact the overall sagittal balance of the spine and PJK might be an early-stage adjustment of the overall balance compensation by the trunk. In severe kyphotic deformity, surgeons pay more attention to achieving satisfactory correction. Excessive forces were applied to the kyphosis area to the reconstruct the sagittal plane, leading to the stress concentration and eventually resulting in the occurrence of PJK.
We also found postoperative PJA＞10° was a significant independent risk factor for PJK. Previously, several studies also detected larger postoperative PJA in the PJK group than the non-PJK group[11, 16]. Patients with postoperative PJA greater than 10° are more likely to develop PJK, which is helpful for early prediction and intervention. We inferred that this might be related to the improper selection of UIV or the insufficient contouring rod during the index surgery.
Although we find no significant differences between the location of UIV and LIV in PJK group and non-PJK group, some previous studies reported different opinions. Pan reported UIV distal to T2 leads to an increasing stress concentration in the junctional area, which may accelerate the occurrence of PJK. Similarly, Watanabe demonstrated that an LIV at or cranial to L3 may change the stress on the spine and then increase the risk of PJK.
Clinical relevance and limitations
The clinical relevance of our study lies in emphasizing the importance of appropriate rod contouring at the proximal junctional area in preventing the occurrence of PJK. During the index surgery, the curvature of the rods should be shaped into a slightly kyphotic profile harmoniously matching the proximal spinal curve when contouring the rods. For patients with hyperkyphosis, the operation of excessive correction of kyphosis leading to the stress concentration on proximal junctional area should be avoided as much as possible. Additionally, for patients with postoperative PJA＞10°, brace treatment will be prescribed.
Our study has several limitations. First, it was a retrospective study with a relatively short follow-up. Second, we were unable to evaluate the impact of PJK on clinical outcomes because patients are too young to fill out questionnaire by themselves. Despite these limitations, all patients were treated by the same surgical strategy and protocol which reduced the impact of the variety of surgical procedures and ensured the homogeneity of surgical results.