Fragility fractures are more likely to occur in patients with PD than in similarly aged non-PD subjects because of a higher risk of falls and lower BMD due to weight loss and lower mobility [8,9]. A meta-analysis in 69,387 patients showed an increased risk of fracture of 2 to 3 times in PD patients compared to controls [10]. A nationwide population-based study in 3,370 PD patients found a significantly increased risk of osteoporosis (hazard ratio 1.32) and surgery for OVC (hazard ratio 2.69) in these patients compared to non-PD subjects [11]. Since osteoporosis and PD tend to develop in elderly people, surgery for OVC in patients with PD is likely to become more common with aging of society. The thoracolumbar spine is the most commonly affected region in OVC, but this condition also affects the lumbar spine. Moreover, clinical and imaging features, and treatment strategies differ in thoracolumbar and low lumbar OVC due to anatomical differences in nerve structures and spinal alignment [2].
PD is a systemic and progressive disease, which suggests that the natural course of PD should be considered in treatment for OVC in a patient with PD. These aspects of PD might also influence clinical features and imaging findings in OVC. Rapid progression and destructive changes of low lumbar OVC in patients with PD, and significantly more PD cases with a cauda equina sign requiring urgent surgery were important findings in this study. Weight loss and sarcopenia are common in PD patients and correlate with greater motor changes, a higher rate of fall, and disease progression [12]. In a cross-sectional study, 55.8% of PD cases had sarcopenia compared to only 8.2% of controls [13]. There is also a strong negative correlation of BMD with the severity of PD, particularly for the lumbar spine [14,15]. Similarly, lower BMI, lumbar YAM, and sarcopenia were present in PD cases in the current study.
In the classification of OVC based on preoperative lateral radiographs, the wedge type (type 1) was common in thoracolumbar OVC, but this type was not found in low lumbar OVC. Flat-type (type 2) low lumbar OVC was dominant in non-PD cases, whereas destructive-type (type 4) was found in most PD cases. Old compression fracture at the thoracolumbar level was found in most non-PD cases, but in few PD cases, but decreased lumbar lordosis before surgery was found in both groups, indicating the nature of PD. Spinal alignment changes may increase middle and/or posterior low lumbar spine loading, and lower bone quality and severe sarcopenia in PD might cause rapid progression and destructive changes of OVC (Fig. 2).
We found a significant improvement in the mean neurological score postoperatively, but there were also relatively high rates of postoperative complications of 45.5% in PD cases and 34.4% in non-PD cases, mainly related to instrumentation. The rate of revision surgery was particularly high in PD cases (27.3%). The higher risks of postoperative complications and revision surgery for patients with PD are well recognized in spine surgery. In a large-scale cohort study of thoracolumbar spine fusion surgery using a national insurance database in the US, PD was significantly associated with an increased risk for medical complications (odds ratio 1.22) and revision surgery (odds ratio 1.70) [16]. Another large-scale matched-pair cohort study using an inpatient database in Japan also suggested that PD was a significant predictor of postoperative complications (odds ratio 1.74) following spine surgery, with delirium being most frequent [17]. In our study, postoperative complications were associated with lower preoperative lumbar YAM, decreased lumbar lordosis, and longer fusion surgery.
In a multicenter study of lumbar spine surgery, surgical failure was more frequent in PD cases in fusion (45.8%) and corrective (67.7%) surgeries than in laminectomy only (33.3%). Thus, lower preoperative lumbar lordosis may be associated with failure of initial surgery, which suggests that use of rigid fixation to achieve and maintain proper lumbar lordosis may be effective in PD cases [18]. We agree with the necessity of long-segment corrective fusion surgery to maintain spinopelvic harmony for global sagittal malalignment, especially such as camptocormia in patients with a stooped posture due to PD progression. However, poorer outcomes and lower fusion rates in multi-level fusion surgery have also been reported for PD cases [19,20], and PD patients treated with long instrumentation, surgery including the thoracic spine, and with less effective spinopelvic realignment tend to require earlier revision [21].
In the present study, lumbar lordosis at follow-up was significantly lower in PD patients, although there was no significant difference in local lumbar lordosis at follow-up between PD and non-PD cases. A biomechanical study showed that a flexion loading condition increased stress by 39.5%-42.7% in the suprajacent disc [22], and adjacent disc stress in longer fusion and fusions involving lower lumbar segments is higher than that in upper lumbar segments [23]. These results suggest that progression of postural instability in the natural course of PD could lead to mechanical stress and instrumentation failure, especially at an upper adjacent level. Most low lumbar OVC cases have decreased lumbar lordosis, but rarely show kyphotic changes due to the low rate of wedge-type collapse [2]. An epidemiological study revealed camptocormia in 4.1% of PD patients [24], and we found that sagittal spinopelvic alignment did not differ in low lumbar OVC cases with and without postoperative complications. Long-fusion surgery should be avoided for patients with single-vertebra OVC because poor sagittal alignment is less common in such cases, but short fusion from a posterior approach cannot correct a spinopelvic mismatch and instrumentation should not be ended in a kyphotic segment.
The surgical indication and strategy should be considered based on the high possibility of deterioration of ADL in the natural course of PD, even if no postoperative complications occur. In general, patients have independent ADL without major difficulties at Hoehn & Yahr stage Ⅱ or below, some restrictions are seen in stage Ⅲ, and independent life becomes difficult at stage Ⅳ or above. PD severity of stage Ⅲ or above has been found to be a significant risk factor for further lumbar spine surgery [25], and the surgical indication for OVC should be considered for patients at stage Ⅲ or below. In assessing long-term outcomes of PD patients, it should also be recognized that the rates of reaching stages Ⅲ and Ⅳ by 5, 10, and 15 years after onset are 30.2% and 6.5%, 57.2% and 27.9%, and 83.5% and 41.2%, respectively [26]. In addition, the rates of dyskinesia, which increases the risk of instrumentation failure, at these time points are 8.4%, 35.1%, and 62.8%, respectively. Appearance of depression decreasing the motivation for rehabilitation after surgery and deterioration of ADL associated with progression of PD is also important in treatment of OVC. Depression can have a significant impact on severity of PD based on health-related quality of life [27]. In our study, 4 of 11 cases (36.4%) had progression of severity of PD at follow-up. Thus, medical management as well as surgical strategy are important to improve the outcomes of spinal surgery in patients with PD.
In surgery for type 2 or 3 low lumbar OVC in non-PD cases, short fusion from a posterior approach is ideal due to easier decompression for the lumbar canal and intervertebral foramen stenosis for fractured vertebra [2]. VP+PS is recommended for cases with cleft formation at the affected vertebra, whereas PS+PLF or PLIF are chosen for cases without cleft formation, in which the affected vertebra often shows sclerotic changes with appearance of intervertebral instability. The optimal surgical strategies for low lumbar OVC in PD cases based on the present study are shown in Figure 3. For most patients with PD, additional reconstruction of the anterior column should be considered, especially for type 4 OVC. Anterior reconstruction using an expandable cage from a lateral lumbar approach and 1-above 1-below fixation might be ideal. In type 2 OVC in PD cases, VP+PS may be more likely to cause instrumentation failure and/or flesh compression fracture than in non-PD cases, due to progression of postural abnormalities and decreased lumbar lordosis in the natural course of PD, which may lead to upper adjacent segment disease and degenerative spondylolisthesis. Because of rare PD cases with sclerotic changes such as in type 3 OVC, PS+PLF or PLIF are often not indicated. VP only (that is, without short fusion) is not recommended in PD and non-PD cases because loading on the middle or posterior lesions of the affected vertebra is increased in low lumbar OVC and collapse of the affected vertebra could easily occur. In most type 2 or 3 cases with vertebral body height loss ≥70%, there is a poor outcome of VP without short fusion [28]. For a few patients with radicular pain or a high risk of instrumentation failure, there may be an option to wait for bone union with aggressive control of PD, and then perform laminectomy.
This study has certain limitations, including its retrospective, single-center design, small number of patients, and lack of use of minimally invasive surgeries, including anterior reconstruction and percutaneous pedicle screw fixation, which have been developed in recent years. Thus, the results may not show true failure and complication rates. Despite these limitations, we believe that our findings provide key insights on management of surgery for low lumbar OVC in patients with PD, since there are few reports on this condition in this patient population.