JRRD

Volume 50, Number 9, 2013
Pages 1177–1186

Association of vitamin D deficiency, secondary hyperparathyroidism,
and heterotopic ossification in spinal cord injury
Christina V. Oleson, MD;1* Benjamin J. Seidel, DO;1 Tingting Zhan, PhD2
Departments of 1Rehabilitation Medicine and 2Biostatistics, Jefferson Medical College of Thomas Jefferson University,
Philadelphia, PA

Abstract—Our objective was to explore the relationshipbetween low vitamin D, secondary hyperparathyroidism, andheterotopic ossification (HO) in patients with spinal cord injury(SCI). Ninety-six subjects with acute or chronic motor complete
SCI participated. Levels of serum vitamin D25(OH), calcium,and intact parathyroid hormone (PTH) were collected, and information regarding nutritional patterns and fracture history was
obtained from subjects. Evidence of current or previous HO wasascertained through chart review. Of the 96 subjects, 12 were
found to have developed HO, 11 with serum vitamin D25(OH)between 5 and 17 ng/mL. Nine subjects exhibited secondary
hyperparathyroidism in the range of 72 to 169 pg/mL. Only one
subject demonstrated HO in the absence of low vitamin D. However, many subjects with low vitamin D (5–31 ng/mL) did nothave hyperparathyroidism or HO. Statistical testing demonstrated a correlation between hyperparathyroidism and HO (p <0.001) as well as hyperparathyroidism and vitamin D deficiency(<20 ng/mL). Direct correlation between HO and low vitamin D
was not observed, but hyperparathyroidism may increase thisrisk. We believe that those patients who demonstrate low vitamin D and elevated PTH should be screened for HO in addition
to beginning vitamin supplementation. Initiating early treatmentof low vitamin D to restore therapeutic levels may prevent
development of HO.

Key words: bone, bone morphogenic protein, bone progenitor
cells, deficiency, heterotopic ossification, hyperparathyroidism,
parathyroid hormone, prevention, spinal cord injury, vitamin D.

1177

INTRODUCTION
The classical definition of heterotopic ossification(HO), set forth by Hsu and Keenan [1], focuses on theformation of lamellar bone in periarticular, nonossified
soft tissues [1–2]. However, HO in its mature form
reveals not only the presence of mature lamellar bone but
also cancellous bone, vessels, and bone marrow with evidence of hematopoiesis [3–4]. New bone forms within
the muscle planes, rather than within the muscle itself,and exists adjacent to the skeleton but exclusive of theperiosteum [2].
HO is characterized by muscle pain, decreased rangeof motion (ROM), and inflammation in the affected

Abbreviations: ALK-P = alkaline phosphatase, BMD = bonemineral density, BMP = bone morphogenic protein, CI = confidence interval, EndMT = endothelial-mesenchymal transition,
FOP = fibrodysplasia ossificans progressive, HO = heterotopic
ossification, PTH = parathyroid hormone, ROM = range of
motion, SCI = spinal cord injury, T = thoracic.
*Address all correspondence to Christina V. Oleson, MD;
Jefferson Medical College of Thomas Jefferson University,
Department of Rehabilitation Medicine, 132 South 10th St,
Regional SCI Center of the Delaware Valley, TJUH, 375
Main Building, Philadelphia, PA 19107; 215-955-6579; fax:
215-955-5152. Email: christinavoleson@msn.com
http://dx.doi.org/10.1682/JRRD.2012.11.0206

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region and is associated with recent trauma and soft tissue injury [1]. The reported incidence ranges from 10 to53 percent depending on the study [5–6]. Some experts
have found that, in terms of patients with traumatic spinal
cord injury (SCI), HO is seen more often among thosewith prolonged immobility, spasticity, deep vein thromboses, or pressure ulcers [7]. Because immobility fromprolonged hospitalization in an intensive care unit, ventilator dependence, and critical illness can all place apatient at risk for any of these conditions, a direct influence of pressure ulcers, spasticity, or venous thromboembolism on development of HO has been challenged [8].
Subbarao and Garrison noted that pressure ulcers andincreased spasticity may be associated with HO yet are
not considered to be causative factors [9]. However, completeness of SCI, the traumatic nature of neurologicalinjury, and direct muscle trauma do appear to be stronger
influencing factors in HO development [1,7].
Although inflammation, completeness of injury, andimmobility following injury may predispose the patient
to heterotopic bone formation, the full scope of the condition is not observed until 1 to 4 mo after initial injury[3,8]. The onset of posttraumatic HO in the setting ofacute SCI is one of several adverse sequelae seen in thesubsequent months following the initial injury. Unlike
other secondary adverse complications of traumatic SCI,
such as pressure ulcers, venous thromboembolism, spasticity, and autonomic dysreflexia, HO has few measuresof effective intervention. Treatment rarely results in complete resolution of the disorder.
In part, the challenge of prevention is related to theimprecise etiology of the condition and the various typesof heterotopic bone. Subbarao and Garrison have proposedfour different types of soft tissue ossification: (1) ossification due to neurologic disorders such as SCI and brain
injury, (2) traumatic myositis ossificans due to blunt injury
or direct contusion to the area, (3) idiopathic HO due toburns or other medical causes, and (4) congenital fibrodysplasia ossificans progressive (FOP) [9]. Whether the sameor different mechanisms of formation exist among the different forms of HO is unknown.
Our group has recently completed an observational
study of vitamin D levels in 42 subjects with acute SCI,
defined as 2 to 6 mo postinjury, and 54 subjects withchronic SCI, defined as >1 yr postinjury [10]. A high percentage of subjects with deficient levels of vitamin D
exhibited HO in bones below the neurological level of
injury. Some had severe deficiency (<13 ng/mL) of vita-

min D with the presence of secondary hyperparathyroidism. This group of subjects serves as the sample of study
in our report. A number of our subjects with chronic SCI
reported having a large bone growth on their hip or knee
several months after SCI but never sought evaluation,
attributing it to inactivity or discontinuing their therapy.
Many did not learn its diagnosis by the name of HO until
their annual SCI visit.
The purpose of our investigation was to examine any
possible relationship between vitamin D deficiency, secondary hyperparathyroidism, and HO formation. Our
specific aims were—
1. To determine whether HO is more common in persons
with hyperparathyroidism and vitamin D deficiency.
2. To determine what minimum serum value of vitamin D
is necessary to reduce risk of HO.

METHODS
Our study was a subproject of a case series examining subjects with acute (2–6 mo postinjury) and chronic(>1 yr postinjury) SCI for possible vitamin D deficiency.
Table 1 shows inclusion and exclusion criteria [11]. All
subjects were cervical level 3 to thoracic (T) level 10,
American Spinal Injury Association Impairment Scale
grades A or B, and between ages 19 and 55. The age
range was derived from another study examining vitaminD levels with focus on the young and middle-aged population. Injuries caudal to T10 were eliminated to prevent
possible inclusion of subjects with conus medullaris or
cauda equina injuries. Both sexes and all racial groups
were invited to participate.
Each subject completed a questionnaire that requested
information about the following: any premorbid diagnosisof osteoporosis; history of other fractures unrelated to theoriginal trauma; history of renal or liver failure or otherliver disease (cirrhosis, hepatitis); medication use, including any seizure medications known to cause osteoporosis
or to impair vitamin D absorption; family history of osteoporosis; and family or personal history of lactose intolerance. Charts were also screened for documentation ofthese conditions. Subjects who consumed more than
400 IU/d of vitamin D as well as those who were prescribed bisphosphonates were not eligible. These criteria
were applied to both subjects with acute and chronic SCI.
Values of serum vitamin D25(OH) and intact parathyroid hormone (PTH) were collected in each subject.

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OLESON et al. Vitamin D deficiency, secondary hyperparathyroidism, and HO in SCI

Table 1.
Criteria for participation (adapted from Oleson et al. [10]).

Criteria
Age (yr)

19–55.

Inclusion
Outside age range.

Exclusion

AIS

C3*–T10 (grade A or B).

Motor incomplete SCI (grade C or D) or neurologic level
below T10.

Vitamin D

Use of multivitamin with 400 IU cholecalciferol
permissible (was given to all acute participants noteating well).

Use of high-potency vitamin D supplements on regularbasis prior to initial assessment. Use of various medications
that deplete vitamin D stores, or total intake from daily supplements exceeds 400 IU vitamin D.

Medical History—

Previous diagnoses of childhood rickets or history of other
adult metabolic bone disorder, including osteoporosis.

Communication

Ability to understand consent process and agree toparticipation. (Witnesses/assistants offered to thoseunable to give signature). Must be able to provide
answers to questionnaire for patient history.

Unable to understand English (consent not available inmultiple languages). Severe cognitive impairment limiting ability to give informed consent. Other condition precluding consent process or questionnaire completion.

*May be on ventilator as long as still able to communicate to give informed consent.
AIS = American Spinal Injury Association Impairment Scale, C = cervical, SCI = spinal cord injury, T = thoracic.

The form of vitamin D chosen for study was serum
D25(OH) since it is the major circulating vitamin D
metabolite and best reflects the overall nutritional statusof the subject, determined by solar and dietary sources[12]. As part of standard of care, subjects with low vitamin D levels, regardless of status of PTH values, wererecommended for treatment. Some chose to pursue treatment at our clinic, while others chose to follow up with
their primary care physician. In cases with severe hyperparathyroidism, subjects were referred to an endocrinologist for more intense treatment than could be provided at
our center. For subjects with data available, results forfollow-up are given.
Inpatient and/or outpatient charts of each subject
were reviewed for reports of any diagnostic studies
revealing the presence of HO or documentation of its
presence in clinic notes. No additional images to screen
for HO were performed for the purposes of this study.
Subjects were also interviewed when data obtained fromchart review were unclear. If during the interview a subject reported decreased ROM in a unilateral hip or knee,
additional images may have been obtained as part ofstandard of care. Subjects were asked about known presence of heterotopic bone and whether they had been seen
for this condition in other hospitals or clinics. No screening for HO was performed in the absence of notable clinical examination findings or apart from a subject’s selfreported joint pain, stiffness, and decreased ROM.

RESULTSOf the 96 subjects who participated in our previous
study [10] on the presence of vitamin D deficiency, 12
reported HO as verified by review of the medical record,
including imaging studies either at the institution of care orat an outside hospital. Of the 78 subjects with low vitamin D(<32 ng/mL), 11 demonstrated HO formation. One patientwith a normal vitamin D value of 52 ng/mL also had HO. Of
the 12 subjects with HO, 9 exhibited marked hyperparathyroidism (PTH range: 71–169 pg/mL). The nine subjects
with elevated PTH all had serum vitamin D levels that were
deficient (<20 ng/mL). Among the subjects with low vitamin D, those with HO had serum vitamin D25(OH) levelsof 5 to 17 ng/mL, while those without HO ranged from 5 to
31 ng/mL. There were 27 subjects with low serum D25(OH)levels between 20 and 31 ng/mL, but no subjects in thisrange had HO.
In all subjects with HO, the location of bone formation was below the neurological level of injury. The mostcommon location was the hip, followed by the knee. In
two subjects, multiple joints were affected. Each subject
with HO indicated that onset of clinical symptoms
occurred after being discharged from acute inpatient
rehabilitation, a duration that ranged from 30 to 65 d.
Chart review indicated that the onset of HO occurred
between 6 and 12 mo postinjury for five patients and overthe first 3 yr for the others, with the exact date of onset
unknown. The twelfth patient, a 48 yr old male with hip

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HO, had normal levels of vitamin D at postinjury years 1
and 2 (46 and 52 ng/mL, respectively), with normal PTH
and calcium levels at the time of interview. His onset
appeared 2 yr postinjury. Two months after HO wasfound, the subject experienced a series of hospitalizations
for weakness and fever and was diagnosed with a paraneoplastic syndrome. This was the only subject of the 96 in
the study who demonstrated HO in the absence of low
vitamin D (<32 ng/mL).
Two subjects among the 78 with low vitamin D (5and 10 ng/mL, respectively) but with presence of HO
failed to reach the level of laboratory-defined hyperparathyroidism of >65 ng/mL. Their PTH values were 40 and
56, respectively. If left untreated for low vitamin D for
additional time, those two subjects may well have demonstrated secondary hyperparathyroidism. Both were in
the second year of their SCI. Of the 18 subjects with normal vitamin D levels (32 ng/mL), all had therapeutic
PTH values and only 1 subject had HO.
Table 2 summarizes the differences found in PTH
levels and HO incidence among those with and without
low vitamin D (serum D25(OH) levels of <32 ng/mL).
Results indicate that presence of HO was correlated withelevated PTH. Vitamin D served as the stratification variable using a threshold of 32 ng/mL, since this represents
the lower limit of the normal range (32–100 ng/mL). The
calculation was performed using a one-sided CochranMantel-Haenszel test, with odds ratio of infinity (95%
confidence interval [CI]: 43.260–infinity, p < 0.001).
However, presence of HO did not correlate specificallywith a vitamin D level below 32 ng/mL by a two-sided
Cochran-Mantel-Haenszel test, with odds ratio 1.953(95% CI: 0.032–39.608, p = 0.51). Although we didobserve an inverse relationship between vitamin D levels
and PTH values, the correlation was not significant using
32 ng/mL as the division between abnormal and normal
vitamin D values (p = 0.20).
Table 3 illustrates our findings for levels of vitamin D
below 20 ng/mL. Similarly, the presence of HO was correlated with elevated PTH using the lower threshold ofserum vitamin D25(OH) of 20 ng/mL as the stratification
variable. This relationship was determined by a one-sided
Cochran-Mantel-Haenszel test, with estimated odds ratio
of infinity (95% CI: 28.193–infinity, p < 0.001). Presenceof HO did not directly correlate with a vitamin D level
below 20 ng/mL either, when calculated by a two-sided
Cochran-Mantel-Haenszel test, with odds ratio 0.554(95% CI: 0.009–11.015, p > 0.99). However, correlation

Table 2.
Cross-tabulation of parathyroid hormone (PTH) and heterotopic
ossification (HO) incidence using low vitamin D threshold (serum
D25(OH) level: 32 ng/mL).

PTH
Level
NormalElevated

Serum D25(OH)<32 ng/mL
HO No
HO Yes67209Serum D25(OH)32 ng/mL
HO No
HO Yes17100Table 3.
Cross-tabulation of parathyroid hormone (PTH) and heterotopic
ossification (HO) incidence using low vitamin D threshold (serum
D25(OH) level: 20 ng/mL).

PTH
Level
NormalElevated

Serum D25(OH)<20 ng/mL
HO No
HO Yes44209Serum D25(OH)20 ng/mL
HO No
HO Yes40100does exist between vitamin D level <20 ng/mL and elevated PTH. The latter finding was determined by a twosided Fisher Exact test, with estimated odds ratio of 0.000(95% CI: 0.000–0.476, p = 0.01).
Our results suggest that the risk of developing elevated PTH appears to have an onset between serum
D25(OH) levels of 20 and 32 ng/mL. While values above
20 ng/mL but below 32 ng/mL may cause mild increases
in PTH, their inclusion may present a confounder whendifferentiating subjects with clearly therapeutic levels(32 ng/mL) from those with clearly insufficient values(<20 ng/mL). In the best interest of patient care, giving
persons with SCI vitamin D to maintain a minimum levelof 32 ng/mL may avoid onset of hyperparathyroidism
that can lead to HO.

FOLLOW-UP TO SUGGESTED TREATMENTFOR SUBJECTS WITH VITAMIN D DEFICIENCY
Available 1 yr follow-up data exists for five of ninesubjects with low vitamin D, laboratory-defined hyperparathyroidism, and presence of HO. Subject 6 had a 9 mofollow-up visit, but the study concluded prior to his 1 yrfollow-up. Subjects were treated with 1,000 IU of cholecalciferol twice daily (oral vitamin D3). All subjects
admitted to partial degrees of noncompliance with medication but felt that they took the pills at least 5 of 7 d of

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the week (72% compliant). Although each subject wasoffered nonsteroidal anti-inflammatory medications fordiscomfort, with appropriate gastrointestinal protection,
most found this medication unsuccessful at treating discomfort from the HO, and compliance was well under
50 percent.
During the follow-up period, no subjects had additional HO formation after initiation of treatment withvitamin D, with or without etidronate. The latter had beenprescribed only for those with HO who had not reachedmaturity. Within 3 mo of starting vitamin D treatment,
the six subjects who presented for follow up had x-ray
reports indicating that maturity of HO had been reached.
Their triple-phase bone scans were no longer active, andPTH values had returned to the normal range in all but
one subject, whose level was 71 ng/mL. Each subject had
serum D25(OH) levels higher than at baseline but under
the lower limit of normal (32 ng/mL).

DISCUSSION
In addressing the specific aims established at the outsetof our study, we found that HO does appear to be morecommon in persons with hyperparathyroidism, but we wereunable to support a direct correlation of HO with vitamin Dlevels. However, vitamin D25(OH) levels <20 ng/mL weresignificantly (negatively) correlated with hyperparathyroidism. While it is unknown what precise value of vitamin D
may place an individual at risk of developing HO (throughits correlation with hyperparathyroidism), we advise maintaining a serum vitamin D25(OH) level 32 ng/mL. Only
one subject with therapeutic vitamin D levels demonstrated
HO, and his condition may have been attributed to a paraneoplastic syndrome.
We had 23 subjects with vitamin D levels between 20
and 31 ng/mL. It could be that subjects with milder
degrees of low vitamin D were identified early enough toprevent secondary hyperparathyroidism. An alternative
explanation is that hyperparathyroidism is a secondary
consequence of low vitamin D, and in our investigation,
was seen only among chronic patients. Because only alimited number of subjects had vitamin D levels between
20 and 32 ng/mL, small sample size may contribute tolack of significant findings in this range.

Rationale for Treatment of Low Vitamin D Levels
Given the large number of medications already takenby those with SCI, both inpatient and outpatient, some
may question the need to treat mild amounts of vitamin D
deficiency. Garland et al. found that 24 percent of bonemineral density (BMD) in the distal femur of subjects
with complete SCI is lost within 4 mo of SCI and 35 percent of BMD is lost within 16 mo [13]. While this earlyphase of SCI contains the period of greatest loss, Bauman
and Spungen and Bauman et al. demonstrated continued
BMD loss at a slower rate in the years following acute
SCI [14–15]. The actual incidence of pathologic fracturesin the population with SCI is approximately 6 percent,but this figure represents those with complete as well as
incomplete SCI, where partial movement is preserved[16]. Those with complete SCI (no motor or sensory
function below the injury level [17]) are estimated tohave a 10-fold greater fracture rate than subjects withincomplete SCI [16]. Given the potential contribution tocontinued osteoporosis following SCI, we advocate supplementation with vitamin D2 or D3 up to the reference
range of 32 to 80 ng/mL.
The Endocrine Society guidelines recommend an
intake of at least 600 to 800 IU oral vitamin D3 for adults
in the therapeutic range but advise considerably higher
doses of 50,000 units of ergocalciferol (vitamin D2)
weekly for 8 wk if subjects are deficient [18]. Concerns
have arisen that persons with SCI who receive vitamin D
supplementation are at increased risk of kidney stones.
Immobilization immediately following acute SCI does
cause hypercalcemia and hypercalciuria [14], but nostudy has shown that supplementation with vitamin D in
SCI increases this occurrence. In addition, several recent
investigations of patients with a tendency for nephrolithiasis, who were treated for coexistent vitamin D deficiency, failed to show a relationship between new stone
formation and vitamin D supplementation [19–21]. Eisner et al. examined 169 patients with variable levels of
serum D25(OH) and found that urinary excretion of calcium was not statistically different among those withvitamin D deficiency (<20 ng/mL), those with vitamin D
insufficiency (20–29 ng/mL), and those with serum
D25(OH) levels 30 ng/mL [19].
Tang et al. studied 16,286 participants in the National
Health and Nutrition Examination Survey [20]. Findings
indicated that serum vitamin D25(OH) levels were notsignificantly different in stone and nonstone formers, andmore specifically, that higher amounts of serum vitamin

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D25(OH) in the 40 to 50 ng/mL range were not associatedwith increased stone formation. While Leaf et al. did findhigher urinary excretion of calcium in 11 subjects amonga small sample of 29 known stone formers, no hypercalcemia was observed and no adverse sequelae arose from theincreased urinary calcium [21]. They advocated that suchpatients be treated for vitamin D deficiency but recommended additional monitoring of 24 h urinary calcium
excretion following supplementation.
In looking at the calcium levels of subjects in our study,
all were within the reference range of 8.5 to 10.5 mEq/dL set
by our clinical laboratory [10]. It appears that the role of
PTH and its interactions with proteins expressed in the posttraumatic setting, such as bone morphogenic protein (BMP),
contribute more to the development of HO than do calcium
levels alone. However, up to now, the mechanism of how
HO forms at the cellular level has largely been speculative.
Low Vitamin D Levels in African Americans
Although normal vitamin D levels have traditionallybeen defined as 32 ng/mL [22–23], Wright et al. recently
proposed that a lower minimum vitamin D value of
20 ng/mL may be adequate in African Americans [24].
Their conclusions are based on examination of large studiesof African-American subjects with osteoarthritis. In thesereports, PTH levels did not demonstrate upregulation
unless serum vitamin D25(OH) levels were <20 ng/mL,
suggesting that perhaps therapeutic values for this group
have a lower limit of 20 ng/mL (range: 20–100 ng/mL)
instead of 32 ng/mL (range: 32–100 ng/mL). Since hyperparathyroidism develops as a secondary effect to low vitamin D, it is appropriate to look at our chronic subjects interms of our previous work [10]. However, we are unable togive clinical data to support the lack of complications inthis borderline range of serum vitamin D in African Americans with chronic SCI, simply because 92.3 percent of subjects had levels less than 20 ng/mL, with just one subject
between 20 and 31 ng/mL and one subject with 32 mg/
mL. Based on uncertainty of the lower limit of normal anda window of normal values extending far above that, we
encourage treatment of low vitamin D for persons of all
ethnicities up to the value of 32 ng/mL, provided that anaffected individual is able to tolerate oral supplementation.
Moreover, we believe that maintaining vitamin Dlevels in the normal range of 32 ng/mL is most desirable
not only for HO prevention but also for many secondary
health benefits as described in our previous work [10].
The current findings also suggest that keeping PTH

within the normal range may prevent HO formation in
the population with SCI. Because low vitamin D levels
can develop within weeks of initial SCI, monitoring both
vitamin D25(OH) levels and PTH levels is highly recommended in the weeks and months following initial SCI.
In addition, annual surveillance of serum vitamin D values is also recommended since low vitamin D is evenmore common in the setting of chronic SCI.
Proposed Mechanisms of Heterotopic Ossification
Formation
Chalmers et al. has suggested that three general conditions are needed for the formation of heterotopic bone:(1) the presence of osteogenic precursor cells, now
believed to be primitive mesenchymal cells; (2) an inducing agent or stimulus that has been identified as BMP(BMP-2 vs BMP-4); and (3) a permissive environment
friendly to osteogenesis [11]. The role of BMP was
explored by Urist, who found that demineralized bonematrix can evoke bone formation ectopically when supplied with BMP [25]. BMP is released from healthy bone
tissue in the settings of immobilization, trauma, venous
stasis, and inflammation. These conditions are consistently observed in the early weeks following traumatic
SCI [1]. Through the actions of BMP, mesenchymal cells
in muscle are altered and transformed into osteoblasts,
which then develop into mature bone cells.
Stover has affirmed that HO begins forming in the intramuscular connective tissue, first through an initial deposition of calcium phosphate, which later ossifies throughdeposition of hydroxyapatite crystals [26]. The hip is themost common joint affected, followed by the knee [26–27].
Elbow and shoulder joints are less commonly affected, as
are smaller joints of the hand [28]. With rare exception,
heterotopic bone is found below the neurological level of
injury [26]. In traumatic SCI, as well as other traumaticconditions, the two primary factors that appear responsible for the etiology of bone deposition at a cellular andmolecular level appear to be the presence of BMPs [29–32] and the ischemic degeneration of involved muscle[33–34]. The sources of BMPs in muscle are mesenchymal cells and endothelial cells; the target cells in the muscle that BMPs act upon are mesenchymal stem cells andsmooth muscle cells [35].
In a series of cell line experiments, Benayahu et al.
observed that when BMP was added to mesenchymal
stromal cells that were multipotent in nature (capable ofdifferentiating into osteoblasts, fibroblasts, adipocytes, or

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OLESON et al. Vitamin D deficiency, secondary hyperparathyroidism, and HO in SCI

other mesenchymal cells), alkaline phosphatase (ALK-P)
activity increased by as much as 10-fold [36]. These premature, mesencymal stem cells from bone marrowstroma have an inherent potential for proliferation andare considered preosteoblastic. In contrast, when BMP
was added to a mature osteoblastic cell line, ALK-P
activity increased by up to fivefold. We found that ALKP activity was increased in both forms of BMP tested,
BMP-2 and BMP-3, with BMP-2 generally exhibiting
more expression.
The same series of experiments was repeated with theaddition of PTH to each of the mesenchymal stromal lines[36]. The premature osteoblastic cell line was noted tohave three times greater ALK-P activity after PTH wasadded. However, PTH did not further increase the ALK-P
activity in the mature osteoblastic cell line. Finally, whenpremature cells supplied with PTH were then challenged
with BMP-2, there was a 12-fold increase in ALK-P
activity. These findings suggest that hyperparathyroidism
and potentially even high normal levels of PTH may theincrease the risk of HO in settings of increased BMP, suchas trauma and immobilization [36]. Apart from trauma, anessential anabolic action of PTH is the enhancement of
osteoblastic activity [37], which may further increase therisk of HO.
One group has recently offered a new theory that
could lead to novel treatments for the condition of HO.
Medici and Olsen have proposed a mechanism of endothelial-mesenchymal transition (EndMT) [38], described
as a form of dedifferentiation of endothelial cells to a stem
cell phenotype. This complex can subsequently form oneof three types of cells: bone, cartilage, or fat. Various promoters or inhibitors that influence this primitive stem cellor a downstream intermediate may direct the ultimate fateof the cell to its destination type. Based on their analysis,
Medici and Olsen propose that vascular endothelial cells
are the leading candidate for the cellular origin of HO andits presence in FOP. In this disorder, 50 percent of bonelesions arise from an endothelial origin, but the source ofthe remaining lesions is unknown. Animal experiments
have recently failed to show a significant contribution
from vascular smooth muscle cells [39–40]. It is unclear
whether a similar mechanism involving cells of endothelial origin applies in patients with SCI. The idea thathuman origins of HO may be different from thoseobserved in previously established animal models offers
new insights on treatment for a condition that has largely
been resistant to pharmacologic intervention.

Other than prescribing etidronate to prevent further
mineralization in established cases of HO [6,35,41–42]
and using nonsteroidal anti-inflammatory agents that arecontraindicated in the immediate postoperative period ofspine fracture stabilization, few interventions have been
attempted. Except in cases of early identification and treatment, limited gains have been achieved from these notedmeasures. This leaves patients with the option of pursuing
surgical resection of the area of HO involved once itmatures, raising a number of concerns in patients who may
have already undergone many procedures for their original
injury. Surgery carries complications of blood loss, superficial or deep wound infection, osteomyelitis, further neurologic or vascular injury, and recurrence of HO [27,43–
46]. Medici and Olsen report that vascular endothelial
growth factor, as well as several EndMT-inducing transcription factors, may serve as potential inhibitors of HO
formation and therefore offer another avenue of potential
treatment for patients affected by HO [39].

CONCLUSIONS
HO appears to be more common in persons with elevated PTH. If vitamin D levels in patients with SCI arefound to be low, supplementation to achieve a goal ofserum vitamin D25(OH) 32 ng/mL should be recommended. PTH levels should be checked with initial andsubsequent vitamin D testing and closely monitored
through the first year after SCI. Maintaining therapeuticserum vitamin D levels will reduce the likelihood ofdeveloping HO by creating a less permissive environment for its formation. Findings in the literature suggestthat BMP-2, combined with PTH in the setting of premature osteoblasts, can increase ALK-P activity. This
upregulation may contribute to the occurrence of HO in
persons with additional risk factors, including immobility, inflammation, and hyperparathyroidism, all commonly
observed in those with traumatic SCI.

ACKNOWLEDGMENTS
Author Contributions:
Study concept and design: C. V. Oleson.
Acquisition of data: C. V. Oleson.
Analysis and interpretation of data: C. V. Oleson, T. Zhan.
Drafting of manuscript: C. V. Oleson, B. J. Seidel.

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Critical revision of manuscript for important intellectual content:
C. V. Oleson, B. J. Seidel.
Statistical analysis: T. Zhan.
Administrative, technical, and material support: C. V. Oleson,
B. J. Seidel.
Study supervision: C. V. Oleson.
Financial Disclosures: The authors have declared that no competinginterests exist.
Funding/Support: This material was based on work supported byintramural support from the University of Alabama at Birmingham
and from Jefferson Medical College of Thomas Jefferson University.
Additional Contributions: The authors wish to thank J. Scott Richards, PhD; Amie B. McLain, MD; Samuel L. Stover, MD; and members of the research staff at the University of Alabama at Birmingham
School of Medicine for their guidance and support of this project.
Institutional Review: Both the original study on vitamin D deficiency
in traumatic SCI [10] and this current study (involving chart review andsubject interview for information about HO) were approved by the Institutional Review Board of the University of Alabama at Birmingham
School of Medicine.
Participant Follow-Up: Participants will not be informed of the publication of this study because the corresponding author no longer works
at the University of Alabama at Birmingham School of Medicine.
Study results will be summarized in clinic handouts that are available
to all patients in their system. However, individual communication is
not anticipated.

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Submitted for publication November 15, 2012. Accepted
in revised form May 28, 2013.
This article and any supplementary material should becited as follows:
Oleson CV, Seidel BJ, Zhan T. Association of vitamin D
deficiency, secondary hyperparathyroidism, and heterotopic ossification in spinal cord injury. J Rehabil Res Dev.
2013;50(9):1177–86.
http://dx.doi.org/10.1682/JRRD.2012.11.0206