Background
Degenerative lumbar spinal stenosis (LSS) is the most common indication for lumbar surgery [
1]. It is treated with decompression, with or without fusion, when surgery is indicated. Most of the patients who suffer from spinal stenosis are 65 years of age or older [
2,
3].
Symptoms of LSS often lead to limited physical activity including walking or prolonged standing [
4,
5], mostly because of leg and/or back pain [
5‐
7]. Osteoporosis and osteopenia are conditions that become more pronounced with age [
8], and they are well known to be influenced by immobilization and low physical activity [
6]. It has previously been reported that women over 50 years of age who undergo spine surgery have a high incidence of osteopenia and osteoporosis: 44.1% and 51.3%, respectively [
9]. Furthermore, osteoporosis and osteopenia are associated with a higher risk of complications such as screw-loosening and fractures in end-instrumented vertebrae [
10‐
13]. The knowledge of a low BMD may also influence the surgical methods for these patients; if instrumentation is used, augmentation may be considered [
14].
Despite the demonstrated association between bone mineral density (BMD) and an increased risk of complications postoperatively [
12], preoperative planning in spine surgery patients does not always include bone quality evaluation. Dual-energy X-ray absorptiometry (DXA) [
15] is the method that is most often used to measure the BMD. A gender- and ethnic group-matched
T-score is calculated and used for the evaluation of osteoporosis/osteopenia [
16]. BMD can be measured in different projections and locations, such as anterior-posterior (AP) or lateral projections of the lumbar spine and anterior-posterior projection of the femoral neck (FN). The projection that is best for evaluating the overall, as well as local, bone quality may vary in different patient groups [
17,
18]. For example, due to facet joint osteoarthritis/osteophytes, which are common in LSS patients, AP lumbar measurements may overestimate the BMD relative to the lateral lumbar projection [
19,
20]. Another frequently used method for indirect evaluation of bone quality is the World Health Organization (WHO) fracture risk assessment tool (FRAX), which focuses on the prediction of future fracture risk. This questionnaire can be used with or without FN BMD [
16,
21]. Another method to measure BMD is quantitative computed tomography (QTC) which exposes the patient to an increased radiation dose compared to DXA [
22].
The aim of the present study was to investigate differences in frequency of osteoporosis/osteopenia, as measured by DXA in different locations and different projections, in a cohort of patients planned for LSS surgery. An age- and gender-matched group of hip osteoarthritis (HOA) patients (with similar anticipated pain and immobilization problems) was used for comparison. Furthermore, we investigated the value of using the FRAX questionnaire, either alone or in combination with FN BMD, to identify spine patients with lumbar osteoporosis/osteopenia. The rationale behind this is that the FRAX can easily be obtained and there is often an FN BMD measurement at the time of referral in LSS patients.
Methods
Patient population
Patients over 50 years of age, who were planned for first-time surgery because of LSS or HOA, were prospectively included in the study during the periods 2013–2014 and 2016 (due to the reorganization of the DXA unit, no patients were included during 2015). Sixty-five patients diagnosed with LSS and of 53 patients with HOA were included in the study. The LSS patients were all planned for decompressive surgery alone or in combination with an instrumented fusion. All HOA patients were planned for total hip replacement. Exclusion criteria were previous surgery (hip or spine) and ongoing medical treatment for osteoporosis and/or rheumatoid arthritis. The study was approved by the regional ethical review board in Gothenburg, Sweden (reference number: 104-11). All the study participants gave their written informed consent.
Dual-energy X-ray absorptiometry (DXA)
All patients underwent bone densitometry with DXA for assessment of BMD 1–2 weeks before surgery. All measurements were performed by the same investigator using the Hologic Discovery™ densitometer (Hologic, Bedford, MA, USA).
BMD was measured in four different areas/projections of the spine and hip: (1) AP lumbar spine projection including the second to the fourth lumbar vertebrae (L2-L4), with separate measurement values for each vertebra; (2) lateral lumbar spine projection (lateral) including L1–L4, with separate measurement values for each vertebra; (3) lateral-middle lumbar spine projection (lat-mid), a smaller area in the middle part of each vertebra in the lateral projection; and (4) the AP femoral neck projection (FN).
T-score values were calculated for all BMD measurements and used for classification of osteoporosis/osteopenia. The WHO definitions for osteoporosis and osteopenia were used, i.e., osteoporosis:
T-score ≤ − 2.5; osteopenia: − 1 >
T-score > − 2.5; and normal:
T-score ≥ − 1 [
16]. This definition has been calculated based on AP spine and FN measurements.
The Z-score was calculated for all measurements and used for comparison between the two groups, since this value compensates for age, gender, and ethnicity.
WHO fracture risk assessment tool (FRAX)
All patients completed the FRAX questionnaire [
23], which includes questions on previous fragility fractures, rheumatoid arthritis, smoking habits, steroid use, alcohol usage, and heredity for hip fractures. Together with information on age, BMI, and gender, a score is created predicting the risk of fractures, such as clinical spine, wrist, proximal humerus, and hip, during the coming 10 years. This score can be calculated with or without the inclusion of FN BMD.
The FRAX questionnaire alone or in combination with DXA FN was compared to the DXA lumbar lateral vertebral measurements. The lumbar lateral vertebral measurements were here considered the best available reference for vertebral bone quality based on previous reports [
19].
Statistics
All results are reported as mean ± SD unless otherwise stated. For group comparisons, Student’s
t test was used and a significance level of
p ≤ 0.05 was used
. Absolute 10-year fracture risk probabilities using the FRAX model alone and FRAX with FN BMD were categorized as low-risk (< 10%), moderate risk (10–20%), and high-risk (> 20%) [
24]. Statistical analysis was performed using the IBM SPSS 22.0 software (IBM Corp., Armonk, NY, USA).
Discussion
In the present study, the occurrence of a T-score ≤ − 2.5 in LSS patients varied between a few percents up to 50%, depending on the projection and location of the DXA measurement. No major differences in the spine or hip DXA measurements were observed in the LSS and HOA patients. Furthermore, by using the FRAX questionnaire in combination with an available DXA of the FN, only 52% of the patients with a T-score ≤ − 2.5 of the spine (measured by lateral spine DXA) could be identified.
The proportion of patients with osteoporosis/osteopenia in this study who required spinal stenosis surgery was in accordance with previous reports. Andersen et al. found 9% osteoporosis and 30% osteopenia using the lumbar AP view in LSS patients (of both genders) [
26]. Lee et al. found 22.6% osteoporosis and 56.6% osteopenia in LSS patients over 60 years (in the AP view), which was considerably higher than in our study. They investigated LSS patients who did not require surgery [
27], which suggests that their patients may have differed from ours in several ways, e.g. higher age and co-morbidities.
Chin et al. examined patients requiring all types of spinal surgery. They found bone density prevalence that was in line with our findings in LSS patients using the lateral lumbar view measurements; 41% of the women had osteoporosis and 46% had osteopenia, [
9] as compared to 49% and 38% in our study.
It is well known that the DXA method gives different results for BMD depending on location and projection. As a consequence, the number of patients who are classified as being osteoporotic can vary widely depending on the type of measurement used. Lumbar AP and FN are the most commonly used projections for measurement of BMD [
17]. In the present study, FN measurements led to the classification of rather few LSS patients as being osteoporotic. We also found that the proportion of patients (both men and women) with LSS who had a
T-score ≤ − 2.5 ranged as much as between 4% for the AP projection (classified as being osteoporotic) and 49% for the lat-mid measurement. One of the reasons for using lateral spine DXA measurements is that osteophytes are very common in the lumbar spine. This may result in “falsely” high BMD values when the AP spine projection is used.
A lower BMD, and therefore a higher incidence of osteoporosis, was seen in the LSS group for the upper/mid lumbar vertebrae than for the lower lumbar vertebrae, which may be explained by more degenerative changes and higher sclerosis in the lower lumbar segments, where spinal stenosis problems are most frequent. This distribution was also seen in the HOA group, indicating that this is not specific to LSS patients but rather reflects the BMD distribution in this age group. Degenerative lumbar spine changes are also frequent in individuals without LSS symptoms. However, for the lowest vertebrae (L4) the mean AP BMD was higher in LSS patients than in HOA patients and the opposite was true for the mean FN BMD, which was slightly higher in the HOA group. This probably reflects the osteophytes and sclerosis in the area of symptoms for the two patient groups and is supported by the fact that L4–L5 is the most common level for spinal stenosis.
To identify the presence of poor bone quality in patients who require spine surgery is important, especially if instrumentation is to be performed and the knowledge of a poor BMD may influence what surgical method that should be chosen. DXA measurement is time-consuming and uses up resources. There are alternative measurement methods to DXA that can be used for BMD measurements such as quantitative computed tomography and DensiProbe Spine [
28]. These methods are less validated and used compared to DXA; however, these may be more sensitive methods [
22]. However, also, these require equipment and personnel resources.
In the present study, we investigated whether the FRAX tool, which is a quick and easy tool used in the assessment of fracture risk and not requiring any specific equipment, would be of any benefit in identifying LSS patients with lumbar osteoporosis. Quite often, these patients already have been screened for osteoporosis with FN DXA, which then could be added to the FRAX questionnaire. By using FRAX in combination with an FN DXA, we were able to identify 52% of the LSS patients with a T-score ≤ − 2.5 of the spine, as evaluated by a lateral spine DXA measurement. Using FRAX alone, we could identify 50%.
The use of FRAX alone or in combination with FN DXA to identify patients with poor lumbar bone quality in the work-up procedure for surgery can therefore be considered to be of doubtful value. However, FRAX is still of value for a general assessment of future fracture risk [
29].
The main limitations of the present study were the limited number of LSS patients included, despite the inclusion of patients over several years. This was due to the fact that many of the patients at our university clinic presented with high comorbidity and/or previous surgery and could therefore not be included. However, few previous studies have compared the different projections of hip and lumbar DXA measurements in LSS patients.