1 Introduction

Oppenheimer's ossicles (OO) are developmental variations in the lumbal part of the vertebral column that are an uncommon and under-recognized source of lower back pain.¹ Albert Oppenheimer identified this variant ossicle in 1942 as an accessory ossicle situated in the interarticular part of the second lumbal vertebra.¹ Multi imaging modalities were applied to delineate this bony ossicle with vertebral arch deformity.² It can be defined as a more frequently encountered variant concerning the non-union that occurs at the tip of the articular process of lumbal vertebrae.^2,3 Patients with OO are frequently asymptomatic.³ However, they may present with lower back pain due to vertebral canal stenosis or nerve compression, and some patients may show painful symptoms which may be confused with fractures, dislocations, traumatic lesions, or developmental abnormalities of the lumbal vertebrae. When it comes to differentiating between a fracture or interarticular defect and an OO, the diagnosis of this developmental variant is of significance.⁴ Clinical findings may not be found in cases where their detection is incidental, while other patients may experience back pain resulting from the ossicle.³ In the general population, the frequency of OO is unknown. The etiology of this unusual ossicle has been debated in the literature, with different theories proposed, such as sesamoid bone, fracture, accessory processes, prior trauma, congenital or impairment in the fusion of ossification centers.⁵ Overall, this narrative review aims to present depth of knowledge regarding the clinical significance of this accessory bone and its effects throughout a wide range of scales.

2 Etiology and pathogenesis

Oppenheimer's ossicle is inherent or occurs following trauma and local degenerative pathology.³ Multiple OO have been described in approximately 20% of patients.^9,12 It has been reported that 73% of symptomatic athletes with additional ossicles exhibited some alterations such as laminae or pedicle hypertrophy, sclerosis, and interarticular abnormalities.¹³ Although there are limited studies inclusive of the general population that reveals or concentrate on the incidence of OO, their presence in plain radiographs of the lumbal part of the vertebral column can be estimated as between 0.5 and 1.5%,¹⁴ or even higher at 13% in cadaveric studies⁹ and 33% in young symptomatic athletes^4,13 (Table 1). As a rough estimate, the incidence rate is more significant in males than in females (6:1) at a prevalence of 1–7%.¹⁵ It has been reported that OO may be responsible for lower back pain, and it may predispose to early degenerative changes in the zygapophysial joint.^16,17 Accessory ossicles, traumatic dislocation, osteoarthritis, synovial cyst, axial spondyloarthritis, rheumatoid arthritis, calcium pyrophosphate deposition syndrome, septic arthritis, and malignant/benign neoplasms are just a few of the diseases that can arise in and around this joint.¹⁸

The congenital absence of the lumbal articular processes should not be confused with the lesions of the vertebral column.¹⁹ Deformity and structural deficiency may be seen at various parts of the laminae, and it might be entirely or partly missing, or the only spinous process may be disrupted with no anomalies in the soft tissues surrounding it, such as spina bifida occulta. A vertebral deficit might combine with superior and inferior articular processes of the lumbal vertebrae, and this infirmity is called spondylolysis. It can result from acute/fatigue displacement or impairment in the developmental process of the lumbal vertebra and stress-induced alterations in the lumbal vertebral arch.¹³ The orientation and size of the zygapophysial joints, bilateral deficits, and facet tropism can cause degenerative spondylolisthesis and unstable vertebral column, leading to neural compression and injury.²⁰ Following a vertebral arch defect existing at this part, this presumably signifies a variety of spondylolysis.¹⁰ The ossicles may be related to deformities of the vertebral arch, among which hypoplasia of the articular process or dysplasia of the zygapophysial joints are counted.^3,12 A laminar bone abnormality, or degenerative variations in the zygapophysial joints, might cause a dislocation in the articular part. Understanding the relationship between osseous abnormalities and degeneration is clinically crucial since degenerative changes in the vertebral column can potentially cause back pain.²¹ Bony defects can be seen in the posterior parts, mainly in the pedicles. In the cervical and upper thoracic parts of the vertebral column, articular surfaces of the zygapophysial joints are more horizontally oriented, allowing for more significant axial rotation and lateral bending. However, the articular surfaces of the zygapophysial joints in the lower thoracic and lumbar parts of the vertebral column are more vertically orientated, allowing limited rotation and lateral bending.²² For this reason, accessory bones in this area can further restrict functions of the vertebral column and cause pain during the movement (Fig. 1).

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Fig. 1 The symptoms, managements, types, location, diagnosis, and associated disorders of Oppenheimer's Ossicle. The narrowed vertebral canal compresses adjacent spinal nerve roots due to Oppenheimer's ossicles (black arrows) in the L1-5 zygapophysial joints. The ossicles are located between the tip of the inferior articular process of L1-5 (black arrows). CT: Computed Tomography, IAP: Inferior Articular Process, L: Lumbal vertebra, MDCT: Multi-Detector Computed Tomography, MRI: Magnetic Resonance Imaging, S: Sacral vertebra, SAP: Superior Articular Process, SPECT: Single-photon Emission Computerized Tomography.

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3 Differential diagnosis

OO is distinguished incidentally, and the ossicles can mimic a defect of the interarticular lumbal part of the vertebral column.^2,4 Their circular shape, smoothness, and location, posterior to the ligamenta flava, are among the features distinguishing them from fractures, osteophytes, and calcified ligaments²³ (Table 2). OO may be mistakenly diagnosed as a fracture or calcified ligamenta flava, therefore, the ossicles must be correctly diagnosed for appropriate treatment.²² The radiographic characteristics are defined as a well-corticated, round, oval, semi-spherical, or triangular ossicle on the inferior articular process.¹⁵ The articular joint surfaces are entirely covered by hyaline cartilage, and their cortex tends to be smooth.⁵ Oppenheimer reported a radiolucent horizontal cleft between the inferior articular processes of the lumbal vertebra and OO, which was approximately 0–1 ​mm in size and had a semi-spherical shape.¹⁶ It may also be seen below the inferior articular processes of the L1-5,^19,24 and it can be related to the articular surface of the joint.⁹ Sometimes this bone is separated from the articular process by a vertical cleft. An asymmetrical enlargement of the two-sided laminae with limited ossification leads to the development of the cleft, and it may relate to the deformation of the vertebral arch. The development of spondylolysis forms a short division of the vertebral arch or a variation at the inferior articular process in a posteroinferior direction.¹⁰

Moreover, the circumferential distinction between the cortical line of the ossicle and intraarticular contrast might also be recognized as a distinctive lucency. Single-photon emission computerized tomography (SPECT) and a multi-detector computed tomography (MDCT) may strengthen these findings more precisely, and these imaging methods are very beneficial for eliminating fracture.^2,4 They can support identifying OO to provide proper diagnosis and appropriate treatment.^2,4 CT or MRI can differentiate the ossicle from degenerative osteophyte or ossification of ligaments in older patients.^3,4 MRI should be employed as the primary diagnosis for cases with back pain and potential stress responses of the interarticular defect of the lumbal region with neurological symptoms.²⁵ Bone scintigraphy and PET are two nuclear imaging methods that can also assess bony abnormalities.¹⁸ In the active athletes with low back pain, bone scintigraphy should be the initial line of thought, followed by CT if bone scintigraphy is positive.²⁵ It can also be observed in anterioposterior, oblique, and lateral radiographs. However, plain radiography might be insufficient to identify the ossicles.⁴

4 Management

Conservative management or surgery is counted among the treatment methods for vertebral canal stenosis caused by OO.⁴ Lower back pain can be managed by analgesics, physical therapy, or zygapophysial joint block⁴ (Fig. 2). Alongside conservative treatment, surgery may be necessary in cases where signs of lumbal stenosis, produced by ossicles, are nonresponsive to conservative treatment. Patients are also affected by ergonomic problems and disability.³² These patients can suffer from lower back pain as the final form of symptoms due to the continuation of synovial lining with the ossicles, which can be treated by facet block for the palliation of pain.³ The treatment process recommended a short course of analgesics using non-steroidal anti-inflammatory drugs and a back muscle strengthening program with lumbal flexion activities.²³ It has been reported that all conservative management should be used before surgical intervention to treat the accessory bones. If surgery is mandatory, postoperative rehabilitation and a thorough workup are required to ensure optimal improvement after surgery.³³ If symptoms do not improve after a diagnostic injection or excision of the ossicles at the lumbarl region, long-term therapy options can be considered, such as percutaneous electrocauterization of the spinal nerve on the affected side of the vertebral column.^2,33 If the presence of OO is neglected, various pathologies such as lumbal disc herniation may occur, and conservative surgical treatment may not be applied.

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Fig. 2 The flowchart of the treatment plan of Oppenheimer's ossicle.

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5 Notable remarks

Oppenheimer's ossicle is an insignificant coincidental finding on the lumbal part of the vertebral column. It mimics interarticular defects, facet fracture, trauma, or fractures of the lumbal vertebra on plain radiographs, but it can be more identified by CT, MDCT, or SPECT. The previous studies reported that OO occurred in the L2-4 of the vertebral column. However, the only report of a female patient with OO occurred in the L5-S1, in which there was a clear fluid-filled space extending to the articular joint. That is probably attributable to the low frequency of OO, which accounts for 0.1–7% of reported cases, and an overlap with degenerative changes, ossification of ligaments, or fracture. Because these conditions affect mainly male patients who face painful syndromes, lower back pain, and have an ossicle in the lower part of the inferior articular process. When distinguishing between irrelevant developmental anomalies of the vertebra, note the additional ossification centers.

Moreover, this accessory ossicle can be separated by a horizontal cleft from the tip of the superior/inferior articular process, and it frequently interacts with the articular surfaces. Surgeons should be aware of OO when examining a patient who complains of asymptomatic pain in the lower back region. Because the radiating lower back pain is caused by compression of lumbal spinal nerve roots, and it may cause numbness or weakness in further stages. It is possible to treat them with conservative management with analgesics and physical therapy. Surgical excision or facet block can be applied if they cause symptoms of lumbar canal stenosis that are refractory to conservative treatment.

6 Conclusion

The current study contributes to the literature underlying the presence of an OO as an accessory bone for undetermined lower back pain and indicates the differential diagnosis and clinical significance of these ossicles. It also highlights that recognizing this ossicle is essential for the proper treatment methods. Knowing this variability is valuable for clinicians when managing asymptomatic or unidentified radiating pain in the lower back region.

Funding/sponsorship

This research did not receive any specific grant from funding agencies in the public, commercial or not-for-profit sectors.

Institutional ethical committee approval

No institutional ethical approval is required for this project.

Author contributions

Conceptualization: EO, DDA, Data curation: EO, OK, DDA, Formal analysis: EO, Methodology: EO, Figure and flowchart: EO, Project administration: EO, Supervision: EO, Writing – original draft: EO, OK, Writing –review & editing: EO.