true or primary coalitions are an aberrant union between two bones caused by a failure of segmentation of the primitive mesenchyme during development.1 acquired coalitions may arise in spastic flatfoots , inflammatory arthritis , intra - articular fractures , osteonecroses , and malignancies.2 
 3 
 4 this malunion can be fibrous ( syndesmosis ) , cartilaginous ( synchondrosis ) , or bony ( synostosis).5 coalitions are predominantly found in the rear foot joints between the talus and calcaneus , calcaneus and navicular , or talus and navicular bones or between the small carpal bones , causing pain or limitation of movement.6 the treatment of coalitions includes temporary immobilization , injections with corticosteroids , or surgical resection.5 
 7 
 8 a coalition between the sacral and the iliac bones has not been described before . although this is the first description of a probably very rare condition , it underlines the importance of a systematic diagnostic process even in cases of apparently  typical  low back pain .
a 33-year - old and otherwise healthy man spontaneously developed a severe left - sided low back pain .
initially , the pain was pronounced after sports activities or physically demanding work . with increasing intensity ,
upon presentation to our clinics , he described a localized pain in the left sacroiliac region with an intermittent nondermatomal radiation into the left thigh .
there was no history of trauma , previous surgery , inflammatory / rheumatoid conditions , or local injections for this pain . on clinical examination , the peripheral sensor and motor functions of the lower extremities were intact , and we found only a localized tenderness to pressure over the left sacroiliac joint . a conventional radiograph ( fig .
1 ) of the pelvis showed normal skeletal features , and the only abnormality was a faint hyperdensity in projection over the superior portion of the left sacroiliac joint .
this lesion was almost missed on the first examination because of its smooth appearance and neighboring bowel gas . to further characterize this lesion , computed tomography ( ct ) ( figs . 2 and 3 ) and magnetic resonance imaging ( mri ; fig .
detailed evaluation of these imaging studies together with experienced skeletal radiologists rendered an inflammatory , a neoplastic , or a post - traumatic origin very unlikely .
morphologically , the lesion most closely resembled a coalition , which was not known to exist at the sacroiliac joint . to confirm this suspected coalition as the source of pain
, a fluoroscopy- and contrast - guided injection with a local anesthetic ( bupivacaine ) was performed via a posterior approach that led to the almost complete resolution of the patient 's symptoms for about 6 hours . 

note the faint hyperdensity in projection over the superior portion of the left sacroiliac joint , which was nearly missed ( arrow ) . 

( a , b ) axial computed tomography of the sacroiliac joints showing bridging bone spurs on the anterior surface of the left joint with a fibrous interruption at the apex .
otherwise , the pelvic structures are without signs of inflammation , osseous destructions , or sequelae of trauma . 
 three - dimensional reconstruction from the computed tomography data depicting l5 , the sacrum , and the adjacent iliac bone .
this anterior view clearly shows the pointed and very localized bone spur bridging the left sacroiliac joint .
the scan resolution is not quite high enough to clearly resolve this , but it can be appreciated that at the apex of the spur there is a noncalcified line running in a superior  inferior direction .
( a , b ) paracoronal t1 and short tau inversion recovery magnetic resonance images of the sacroiliac joints .
the bone marrow shows normal signal intensity without any bone marrow edema or fatty deposits .
anteriorly , the bone spurs can be seen ( arrow ) . because the conservative treatment with physiotherapy and a range of analgesics had failed , the patient was offered a surgical therapy .
the resection of the coalition was performed via an anterior retroperitoneal approach and through the tissue plane between the psoas and the iliacus muscles .
technically , an osteotome was used to remove the bone spurs flush with the cortex of the surrounding bone , and the open pores of the resection footprint were sealed with a bone wax .
the patient 's pain resolved after surgery , and the only complication was a postoperative hypoesthesia in the l4 dermatome that resolved within 6 months after the surgery .
the histopathologic examination showed a predominantly laminar dense haversian - type bone with an inconspicuous osteoblastic lining and a minimal orthoclastic activity bridging a central , comparatively thin fibrocartilaginous component , similar to the  bar  known from coalitions of the foot ( figs . 5a and 5b ) .
( a , b ) histologic examination showed predominantly laminar dense haversian - type bone with a central fibrocartilaginous component .
overview : hematoxylin and eosin staining , 10 ; magnified image : elastica van gieson staining , 100 .
follow - up visits were scheduled at 3 , 6 , 8 , and 12 months .
the patient remained completely pain - free and returned to work and sports activities without limitations . at 8 months , a follow - up ct scan showed the complete removal of the coalition without any signs of recurrence ( fig .
the main differential diagnosis that we considered during the diagnostic workup was bridging osteophytes secondary to an injury or to an infectious / inflammatory process . as mentioned previously ,
the patient 's history was completely devoid of any hints toward such a pathogenesis , even on targeted repeat questioning .
the sacroiliac joints were completely normal on ct as well as on mri , and the lesion was much more localized than degenerative changes usually are . in our case ,
a fibrous bar was suspected from the mri , which is very typical for a coalition .
we can only speculate that the slowly increasing calcification of this bar with age and its consequently increasing effect on sacroiliac joint kinematics caused the onset of the symptoms .
the complete resolution of symptoms after resection and the absence of any local recurrence at 12 months further corroborate the diagnosis .
 
 study design  case report . 
 
 objective  this report describes the first case of a sacroiliac coalition , its clinical features , the diagnostic difficulties , and the surgical treatment chosen in this case . 
 
 methods  a 33-year - old man presented to our outpatient clinics complaining of severe left - sided low back pain with an intermitted nondermatomal radiation into the left thigh . 
 the only abnormality on a pelvic radiograph was a coin - size , faint hyperdensity , which was almost overlooked . 
 subsequent computed tomography ( ct ) and magnetic resonance imaging ( mri ) confirmed two bridging bone spurs on the anterior surface of the left joint with a fibrous interruption at the apex . after the conservative treatment failed , a surgical treatment was offered to the patient . 
 the coalition was resected via an anterior retroperitoneal approach and through the tissue plane between the psoas and the iliacus muscles . 
 a histopathologic examination was performed and confirmed the diagnosis of a coalition . 
 
 results  the patient 's pain resolved immediately after surgery . 
 after 8 months , a follow - up ct scan showed complete removal of the coalition without any signs of recurrence , and at 12 months ' follow - up , the patient remained pain - free . 
 
 conclusions  this is the first published case of a sacroiliac coalition . the only sign of this rare condition on the plain radiographs was very easy to miss . 
 as has been discussed in the literature , ct and mri are important in the differential diagnostics of such lesions . the histopathologic findings included that of a fibrous bar , confirming the diagnosis , which is further corroborated by the complete resolution of the symptoms . 
Introduction
Case Report
Discussion