We report the first morphological characterization of the pedicles and surrounding structures from C3 to C6 in patients with VAD, and analyze correlations among these structures to provide an anatomical basis for safer and more accurate CPS insertion or appropriate choice of internal fixation in this patient group. The prevalence of VAD (21.8%) was lower than in previous studies (from 38.5 to 73%) [13,14,15], this was possibly due to differences in inclusion criteria, exclusion criteria, or VAD definition. Most VAD (69.1%) was found on the left side, which is consistent with previous findings .
POW is the most important factor influencing the safety and feasibility of CPS insertion. According to several previous studies, the lateral wall is the thinnest of the cervical pedicle walls [12, 19]. Therefore, it is the easiest to breach during CPS insertion and so cause the potential risk of VAI [6,7,8]. In this study, POW was significantly smaller on the dominant side than the non-dominant side (P < 0.001). Furthermore, POW was more frequently < 4 mm on the dominant side compared to the non-dominant side (P < 0.001), indicating that difficult or unfeasible CPS insertion is more common on the dominant side in VAD patients [12, 20]. Moreover, this may confer a greater potential risk of pedicle wall breach on the dominant side than the non-dominant side in VAD patients. However, several studies reported that lateral wall breach or even mild intrusion into the TF did not result in a higher VAI rate [7, 8, 21]. We suggest that a “safe space” between the VA and pedicle lateral wall, the DPVA, may account for above mentioned findings. Additionally, DPVA is a more specific and direct value than ORTF, so we combined POW with DPVA, and examined POW + DPVA < 5 mm as an assessment parameter in further analysis. Despite larger DPVA on the dominant side (P < 0.001), the frequency of POW + DPVA < 5 mm was also higher on the dominant side (P = 0.006), suggesting that the bilateral difference in DPVA is insufficient as a metric to guide the choice of CPS insertion. Nonetheless, both POW and POW + DPVA measures suggest a higher risk of pedicle border breach and ensuing VAI on the dominant side than the non-dominant side.
POW < 4 mm was found more frequently at C3 and C4 on both sides but was most frequent on the dominant side (P < 0.001). Similarly, POW + DPVA < 5 mm was most common at C3 and C4, so surgeons should be more careful in choosing CPS treatment at these levels for patients with VAD. With the development and application of new surgical navigation methods, the ability to accurately identify the best CPS insertion position and angle has significantly improved [21,22,23,24]. However, smaller POW and DPVA are both indicative of VAI risk. Therefore, we do not recommend CPS insertion at pedicles with POW < 4 mm or POW + DPVA < 5 mm, especially on the dominant side due to the potential of VAI.
Despite smaller POW on the dominant side, POW showed no direct correlation with DVA or AVA on the dominant side, although it was correlated with DVA and AVA on the opposite side. POW was, however, correlated with ATF and the latter was correlated with DVA or AVA on both sides. These results suggest that POW on the dominant side is influenced directly or to a greater extent by ATF than by DVA or AVA. We speculate that VAD may indirectly affect the development of the pedicle. During the embryonic stage, the VA develops before the TF. Therefore, VAD may have a stronger influence on TF than a normal VA, and the larger TF on the dominant side may further affect the development of the ipsilateral pedicle. This indirect influence on the pedicle may not manifest as a correlation between POW and DVA or AVA. Additionally, in our study, 20 cases exhibited pathway variation for TF entry at C5, and 85% of these cases had a nearly closed TF, which supports this notion.
DPVA was correlated with ATF on the dominant side but not on the non-dominant side. This further supports the greater developmental influence of VAD. In our study, DVA, AVA, and ATF were significantly larger on the dominant side and gradually increased from C3 to C6. Additionally, the increase in ATF at lower cervical levels appeared larger on the dominant side. This tendency was reflected by a decline in ORTF (AVA/ATF) at lower levels (Fig. 3F). Further, this difference in ATF increase rate compared to both DVA and AVA would influence DPVA and POW on the dominant side. Therefore, the development of the TF may be a key factor influencing the final DPVA and POW, especially on the dominant side. In general, the correlations between all parameters were not strong; more clear correlations between them needs further study with larger sample size in the future.