Comparison of CT versus MRI measurements of transverse atlantal ligament integrity in craniovertebral junction injuries. Part 1: A clinical study

Utility of Anterior Atlantodens Interval Widening on Cervical Spine CT for Assessing Transverse Atlantal Ligament Injury

STUDY DESIGN

Retrospective, cross-sectional.

OBJECTIVES

To identify trauma patients with confirmed tears of the transverse atlantal ligament on cervical MRI and measure several parameters of atlanto-axial alignment on cervical CT, including the anterior atlantodens interval, to determine which method is most sensitive in predicting transverse atlantal ligament injury.

METHODS

Adult trauma patients who suffered a transverse atlantal ligament tear on cervical MRI were identified retrospectively. The cervical CT and MRI exams for these patients were reviewed for the following: anterior and lateral atlantodens interval widening, lateral C1 mass offset, C1-C2 rotatory subluxation, and transverse atlantal ligament injuries on cervical MRI.

RESULTS

Twenty-six patients were identified with a tear of the transverse atlantal ligament on cervical MRI. Twelve percent of these patients demonstrated an anterior dens interval measuring greater than 2 mm, 26% of patients demonstrated lateral mass offset of C1 on C2 (average offset of 2.4 mm), 18% of patients demonstrated an asymmetry greater than 1 mm between the left and right lateral atlantodens interval, and one patient demonstrated atlanto-axial rotation measuring greater than 20%. Ten patients had an accompanying C1 burst fracture and eight patients had a C2 fracture. One patient demonstrated widening of the atlanto-occipital joint space greater than 2 mm indicative of craniocervical dissociation injury.

CONCLUSIONS

An anterior atlantodens interval measuring greater than 2 mm is an unreliable methodology to screen trauma patients for transverse altantal ligament injuries and atlanto-axial instability. Moreover, C1 lateral mass offset, lateral atlantodens asymmetry, and atlanto-axial rotation were all poor predictors of transverse atlantal ligament tears.

Assessment of Cervical Skeletal Trauma: The Synergistic Contribution of Forensic and Clinical Medicine to a Case of Corpse Concealment

Fatal neck injuries represent a major challenge in forensic pathology because the anatomical complexity and high variability of neck structures make it often extremely difficult to differentiate true pathological findings from artifacts at autopsy. This topic becomes even more relevant when the forensic pathologist is required to make a pathophysiological evaluation of bone fractures in the absence of soft tissue to support the diagnosis. We report a case of unidentified, stone-covered, skeletonized human remains found within a pit below an abandoned building with bony lesions of the cervical spine and ribs, including a full-thickness fracture of the right lateral mass of the atlas (C1). After a careful study of the fractures was carried out by screening forensic literature and anthropological studies, clinical neurosurgical expertise was called upon to provide a reliable explanation. A rapid and violent twisting of the neck in the opposite direction from the fracture site by an attacker who pinned the victim's torso is the scenario that most likely occurred in our case. This case report shows that the diagnosis of cervical spine injuries in skeletal remains should be the result of a multidisciplinary approach that integrates forensic, anthropological, and clinical expertise.

Technical Considerations in Surgical Fixation of Jefferson Fracture

Jefferson fracture is defined as the simultaneous disruption of the continuity of the anterior and posterior arches of the atlas vertebra. It generally results from an axial impact to the head. Most of these fractures are amenable to nonoperative management. Significant disruption of the transverse atlantal ligament that is the main stabilizing ligament of the atlantoaxial articulation and contiguous spinal injuries often form the indications for operative intervention in these fractures. The outward and caudal displacement of the C1 lateral masses observed in these fractures often requires significant deviation from the standard operative technique of atlantoaxial fixation when the osseous elements are intact. Accordingly, we have described the surgical nuances relevant to the exposure and instrumentation of the atlantoaxial region in the setting of Jefferson fracture, through our experience in two cases.

Risk Factors for Fracture Nonunion and Transverse Atlantal Ligament Injury After Isolated Atlas Fractures: A Case Series of 97 Patients

BACKGROUND

The management of atlas fractures is controversial and hinges on the integrity of transverse atlantal ligament (TAL).

OBJECTIVE

To identify risk factors for atlas fracture nonunion, with and without TAL injury.

METHODS

All isolated, traumatic atlas fractures treated at our institution between 1999 and 2016 were analyzed. Multivariable logistic regression was used to identify variables associated with TAL injury confirmed on MRI, occult TAL injury seen on MRI but not suspected on computed tomography (CT), and with fracture nonunion on follow-up CT at 12 weeks.

RESULTS

Lateral mass displacement (LMD) ≥ 7 mm had a 48.2% sensitivity, 98.3% specificity, and 82.6% accuracy for identifying TAL injury. MRI-confirmed TAL injury was independently associated with LMD > 7 mm ( P = .004) and atlanto-dental interval ( P = .039), and occult TAL injury was associated with atlanto-dental interval ( P = .019). Halo immobilization was associated with having a Gehweiler type 3 fracture ( P = .020), a high-risk injury mechanism ( P = .023), and an 18.1% complication rate. Thirteen patients with TAL injury on MRI and/or LMD ≥ 7 mm were treated with a cervical collar only, and 11 patients (84.6%) healed at 12 weeks. Nonunion rates at 12 weeks were equivalent between halo (11.1%) and cervical collar (12.5%). Only age independently predicted nonunion at 12 weeks ( P = .026).

CONCLUSION

LMD > 7 mm on CT is not sensitive for TAL injury. Some atlas fractures with TAL injury can be managed with a cervical collar. Nonunion rates are not different between halo immobilization and cervical collar, but a strong selection bias precludes directly comparing the efficacy of these modalities. Age independently predicts nonunion.

Surgical Versus Conservative Management for Treating Unstable Atlas Fractures: A Multicenter Study

OBJECTIVE

This multicenter study compared radiological parameters and clinical outcomes between surgical and nonsurgical management and investigated treatment characteristics associated with the successful management of unstable atlas fractures.

METHODS

We retrospectively evaluated 53 consecutive patients with unstable atlas fracture who underwent halo-vest immobilization (HVI) or surgical fixation. Clinical outcomes were assessed using neck visual analogue scale and disability index. The radiological assessment included total lateral mass displacement (LMD) and the anterior atlantodental interval (AADI).

RESULTS

Thirty-two patients underwent surgical fixation and 21 received HVI (mean follow-up, 24.9 months). In the surgical fixation, but not in the HVI, LMD, and AADI showed statistically significant improvements at the last follow-up. The osseous healing rate and time-to-healing were 100% and 14.3 weeks with surgical fixation, compared with 71.43% and 20.0 weeks with HVI, respectively. Patients treated with HVI showed poorer neck pain and neck disability outcomes than those who received surgical treatment. LMD showed an association with osseous healing outcomes in nonoperative management. Clinical outcomes and osseous healing showed no significant differences according to Dickman's classification of transverse atlantal ligament injuries.

CONCLUSION

Surgical internal fixation had a higher fusion rate, shorter fracture healing time, more favorable clinical outcomes, and a more significant reduction in LMD and AADI compared to nonoperative management. The pitfalls of external immobilization are inadequate maintenance and a lower probability of reducing fractured lateral masses. Stabilization by surgical reduction with interconnected fixation proved to be a more practical management strategy than nonoperative treatment for unstable atlas fractures.

Fifty years later: the "rule of Spence" is finally ready for retirement

Determination of the optimal approach to traumatic atlas fractures with or without transverse atlantal ligament (TAL) injury requires a nuanced understanding of the biomechanics of the atlantoaxial complex. The "rule of Spence" (ROS) was created in 1970 in a landmark effort to streamline management of burst-type atlas fractures. The ROS states that radiographic evidence of lateral mass displacement (LMD) (i.e., the distance that the C1 lateral masses extend beyond the C2 superior articular processes) greater than 6.9 mm may indicate both a torn TAL and need for surgical management. Since then, the ROS has become ubiquitous in the spine literature about atlas injuries. However, in the decades since the original paper by Spence et al., modern research efforts and imaging advancements have revealed that the ROS is inaccurate on both fronts: it neither accurately predicts a TAL injury nor does it inform surgical decision-making. The purpose of this review was to delineate the history of the ROS, demonstrate its limitations, present findings in the existing literature on ROS and LMD thresholds, and discuss the current landscape of management techniques for TAL injuries, including parameters such as the atlantodental interval and type of injury according to the Dickman classification system and AO Spine upper cervical injury classification system. The ROS was revolutionary for initially investigating and later propelling the biomechanical and clinical understanding of atlas fractures and TAL injuries; however, it is time to retire its legacy as a rule.

The relationship between radiologic parameters and transverse atlantal ligament injury obtained from MRI scans in patients with an isolated atlas burst fracture: A retrospective observational study

The treatment of an atlas burst fracture depends on whether transverse atlantal ligament (TAL) injury is present. We compared the radiologic parameters associated with the presence of a TAL injury as detected using magnetic resonance imaging (MRI), and verified whether the lateral mass displacement (LMD) criteria currently used to diagnose TAL injuries in atlas burst fractures are reliable or need revision.Thirty patients who presented with isolated atlas burst fractures were included in this retrospective observational study. We measured radiologic parameters, including LMD, atlanto-dental interval, basion-dens interval, internal lateral mass displacement, and external lateral mass displacement, in each patient at the time of initial presentation. The presence of TAL injury was evaluated using MRI. We compared the radiologic parameters and characteristics of patients who presented with TAL injury. We also determined the sensitivity and specificity of an LMD test to accurately diagnose TAL injury based on MRI. Finally, we compared the radiologic parameters according to the presence of surgical treatment and patient union status.Twenty patients presented with an intact TAL, while 10 patients had a TAL injury on MRI. LMD was significantly higher in patients with TAL injury (9.61 vs 3.73 mm, P < .001). In multivariable logistic regression analysis, LMD was also significantly higher in patients with TAL injury. The sensitivity and specificity of LMD for diagnosing TAL injury based on MRI in patients with isolated C1 fractures were 90% and 100%, respectively. The incidence of an LMD greater than 8.1 mm was statistically higher in patients than in those without TAL injury (90% vs 0%, P < .001).Nine patients underwent surgery for isolated atlas burst fractures, and 21 did not. LMD (9.56 vs 4.03 mm, P < .001) and fracture gap (7.96 vs 4.01 mm, P < .001) were significantly higher in patients who underwent surgery.Among the various radiologic parameters, LMD closely correlated with the presence of TAL injury, as patients with an LMD greater than 8.1 mm were more likely to have a TAL injury in the case of atlas burst fractures. LMD is a good method for predicting the presence of TAL injury if MRI is not available.

[Cervical fractures and disc lesions]

CLINICAL ISSUE

The head accounts for about 8% of the total body weight, and only modest ligaments stabilize the cervical spine. In children, the ratio head weight/body mass is even worse, so not surprisingly injuries to the cervical spine are common. This article reviews the most common classifications of different cervical fractures. In addition, ruptures of the ligaments and lesions to the intervertebral discs and the vertebral arteries are discussed.

PRACTICAL RECOMMENDATIONS

In high velocity trauma, it is vital to exclude lesions to the vertebral arteries and the cervical ligaments to prevent/minimize further harm and to accurately assess the stability of the cervical spine.

Risk Factors for Transverse Ligament Disruption and Vertebral Artery Injury Following an Atlas Fracture

BACKGROUND

Atlas fracture occurs in 3%-13% of all cervical spinal injuries and is often associated with other injuries. The factors associated with concomitant transverse ligament disruption and vertebral artery injury remain underexamined.

METHODS

We retrospectively reviewed 97 consecutive cases of atlas fractures. We analyzed demographic and clinic characteristics, including mechanism of injury, fracture type, and associated injuries. We identified factors independently associated with vertebral artery injury and/or transverse ligament disruption.

RESULTS

On multivariable analysis, vertebral artery injury was independently, positively associated with injury to the transverse ligament (odds ratio [OR], 8.51 [1.17, 61.72], P = 0.034), associated facial injury (OR, 7.78 [1.05, 57.50]; P = 0.045), intoxication at presentation (OR, 51.42 [1.10, 2408.82]; P = 0.045), and negatively associated with type 3 fractures (OR, 0.081 [0.0081, 0.814]; P = 0.033). There was a trend toward a positive association with a violence mechanism of injury (OR, 33.47 [0.75, 1487.89]; P = 0.070). Transverse ligament injury was independently associated with other injuries to the spine (OR, 13.07362 [2.43, 70.28]; P = 0.003), atlantodental interval (OR, 2.63 [1.02, 6.75]; P = 0.045), lateral mass displacement (OR, 1.78 [1.32, 2.39]; P < 0.001), and male sex (OR, 7.07 [1.47, 34.06]; P = 0.015). There was a trend toward a positive association with injury to the vertebral artery (OR, 5.13 [0.96, 27.35]; P = 0.056).

CONCLUSIONS

Among patients with atlas fractures, vertebral artery injury and transverse ligament disruption are associated with each other. Mechanism of injury, fracture type, and intoxication at the time of injury were associated with vertebral artery injury, and atlantodental interval and lateral mass displacement are associated with magnetic resonance imaging-confirmed injury to the transverse ligament.

[Guideline-conform diagnostics for injuries of the subaxial cervical spine]

Injuries to the subaxial cervical spine are increasing and have a high risk for neurological injury in comparison to the thoracic and lumbar spine. The current treatment recommendations according to the recommendations of the section spine of the German Society for Orthopaedics and Trauma (DGOU) and the S1 guidelines of the German Society for Trauma Surgery are summarized in this article. High-energy as well as low-energy trauma can cause a significant injury to the cervical spine. If there is a suspicion of a cervical spine injury, a tomographic imaging modality (CT/MRI) is the procedure of choice. Injuries should be classified according to the AOSpine classification for subaxial injuries. Based on this classification, a decision on a conservative or operative treatment regimen as well as individual details of the treatment can be made.

Objective Indirect Assessment of Transverse Ligament Competence Using Quantitative Analysis of 3-Dimensional Segmented Flexion-Extension Computed Tomography Scan

OBJECTIVE

Assessment of transverse ligament (TL) competence in patients with suspected atlantoaxial instability is performed via indirect radiograph measurements or direct TL visualization on magnetic resonance imaging (MRI). Interpretation of these images can be limited by unique patient anatomy or imaging technique variability. We report a novel technique for evaluating TL competence using flexion-extension computed tomography (feCT) scan with 3-dimensional (3D) segmentation and quantitative analysis.

METHODS

feCT scans of 11 patients were segmented to create 3D surface models. Six patients with atlantoaxial pathology were evaluated for possible instability based on clinical examination and imaging findings. The other 5 patients had no clinical or imaging evidence of atlantoaxial injury. Dynamic atlantodental interval (ADI) was calculated using point-to-point voxel changes between flexion and extension 3D models. Magnitude and direction of ADI changes were quantified and compared with available cervical spine flexion-extension radiograph and/or MRI findings.

RESULTS

In the 5 patients without evidence of atlantoaxial injury, 94.3% of ADI vector changes were <3.0 mm. In the 3 patients with atlantoaxial pathology but TL competence, 92.4% of ADI vector changes were <3.0 mm. In the 3 patients with atlantoaxial pathology and TL incompetence, only 49.1% of ADI vector changes were <3.0 mm. In addition to the significant atlantoaxial subluxation in these 3 patients, there was significant rotational motion compared with the patients with an intact TL.

CONCLUSIONS

3D segmentation and quantitative analysis of feCT scan allow objective indirect assessment of TL integrity. Results are consistent with MRI findings and offer additional biomechanical information regarding the direction and distribution of atlantoaxial motion.

Radiologic criteria to predict injury of the transverse atlantal ligament in unilateral sagittal split fractures of the C1 lateral mass

Unilateral sagittal split fracture (USSF) of the C1 lateral mass (LM) has been recently recognized as a rare variant of C1 atlas fracture. To date, there has been no study to investigate whether radiologic criteria can be applied to determine the presence or absence of transverse atlantal ligament (TAL) injury in USSF of the C1 LM.Twenty six consecutive cases of USSF of the C1 LM were included in this study. According to Dickman classification, 16 cases were TAL injury, and 10 cases were TAL intact. Radiologic parameters were measured and compared between the 2 groups.Total LM displacement (LMD) of the 2 sides (5.9 ± 2.0 mm vs 1.2 ± 2.0 mm), unilateral LMD of the fracture side (4.3 ± 1.2 mm vs 1.0 ± 1.1 mm), atlanto-dental interval (ADI) (2.0 ± 0.9 mm vs 1.5 ± 0.4 mm), and fracture gap (6.9 ± 2.7 mm vs 2.1 ± 1.1 mm) were statistically higher in the TAL injury group than the TAL intact group. However, basion-dental interval, clivus canal angle, and atlanto-occipital joint axis angle were not different between the 2 groups. Total LMD and unilateral LMD positively correlated with ADI and fracture gap. The incidence of fracture gap larger than 7 mm was statistically higher in the TAL injury group than the TAL intact group (81% vs 30%).In conclusion, total LMD > 5.9 mm or unilateral LMD > 4.3 mm suggests the presence of TAL injury in USSF of the C1 LM. The possibility of diagnostic error for TAL injury can be further reduced in USSF of the C1 LM by considering the fracture gap larger than 7 mm.

C1:C2 ratio is a potential tool assessing atlas fracture displacement and transverse ligament injury

OBJECTIVES

The aim of this study was to determine the reliability of a C1:C2 ratio in a cohort of patients with atlas fractures. Second, we aimed to consider the utility of the C1:C2 ratio with regard to diagnosis of transverse ligament (TL) injury.

DESIGN

This is a retrospective analysis.

METHODS

Patients with atlas fractures in the Waikato region between 2008 and 2010 were identified retrospectively through clinical coding and collated radiology trauma database.

MAIN OUTCOME MEASUREMENTS

The maximal width of C1 and C2 was measured using the first-taken trauma radiograph series. Combined overhang of lateral masses (△mm) and a C1:C2 ratio was then calculated. Final ratio and atlanto-dens interval (ADI) were measured at the last clinical follow-up.

RESULTS

A total of 24 patients with full radiographic records were included. Of these, five patients (21%) had TL injuries confirmed on computed tomography or magnetic resonance imaging. No patient with a ratio 1.15 had an intact TL, whereas a ratio of >1.10 captured 80% of TL injuries. The ratio (P < 0.001) and delta values (P < 0.001) were statistically significantly different between TL-injured and TL-intact cohorts. Two patients in the TL injury group demonstrated increased ADI on final follow-up with a ratio of >1.10.

CONCLUSIONS

A C1:C2 ratio >1.10 on plain radiographs showed a sensitivity of 80% in detecting atlas fractures with associated TL injury. All patients with a ratio of ≥1.15 had TL rupture subsequently confirmed by an advanced modality. A ratio calculation on radiographs is a potentially useful method of describing atlas lateral mass displacement.

LEVEL OF EVIDENCE

Level III.

"Rule of Spence" and Dickman's Classification of Transverse Atlantal Ligament Injury Revisited: Discrepancy of Prediction on Atlantoaxial Stability Based on Clinical Outcome of Nonoperative Treatment for Atlas Fractures

Detailed clinical information of 13 adult patients with acute atlantal fractures underwent nonoperative treatment was retrospectively studied. "Rule of Spence" was found inaccurate in predicting either integrity of transverse atlantal ligament (TAL) or atlantoaxial stability, whereas Dickman's classification of TAL injury was more superior to "rule of Spence" on both prediction.

STUDY DESIGN

A retrospective study.

OBJECTIVE

To evaluate the prediction accuracy of "Rule of Spence" and Dickman's classification of the transverse atlantal ligament (TAL) injury on clinical outcomes (mainly focused on atlantoaxial stability) of atlas fractures treated nonoperatively.

SUMMARY OF BACKGROUND DATA

TAL is regarded as primary stabilizer of the atlantoaxial complex. Atlas fractures are categorized as unstable and stable according to TAL injury or not. "Rule of Spence" and Dickman's classification have been widely used to evaluate the integrity of TAL indirectly or directly. However, there is controversy about how to interpret and apply these image measures appropriately in treatment decision making, and comparing the two measures in same cohort has been lack.

METHODS

From January 2013 to December 2015, 13 adult patients with atlas fractures, treated nonoperatively at acute posttraumatic phase and followed up for at least 2 years, were enrolled in the study. Lateral mass offset (LMO) and TAL injury were measured by radiography. Atlantoaxial stability, pain in occipital region, limitation of cervical motion, neurological dysfunction, and quality of daily life were evaluated as clinical outcomes.

RESULTS

LMO less than 6.9 mm was inaccurate either to exclud TAL injury (4/8, 50% failed) or to predict clinical outcomes (2/8, 25% failed), whereas LMO greater than 6.9 mm was accurate to determine TAL injury (5/5, 100% succeeded) but not to predict atlantoaxial stability (4/5, 80% failed). Two cases with Dickman's classification type I injury (100%) failed to restore C1-2 stability and six of seven type II (85.7%) succeeded. Three patients were indicated for fusion surgery due to instability, and one due to traumatic arthritis. Overall clinical outcomes were satisfactory as pain and quality of life were considered.

CONCLUSION

Dickman's classification of TAL injury is of higher superiority to "Rule of Spence" in term of the accuracy of predicting atlantoaxial stability of nonoperatively treated atlas fractures.

LEVEL OF EVIDENCE

4.

Odontoid fracture with missed diagnosis of Transverse Atlantal Ligament (TAL) injury resulting in late-onset instability

Background:

Concurrent injuries to both the odontoid and transverse atlantal ligament are rare and can be easily missed. Failure to diagnose both lesions potentially leads to the late onset of sagittal plane instability and acute myelopathy. Here, we present a patient with an odontoid fracture whose transverse atlantal ligament (TAL) injury was originally missed on magnetic resonance imaging (MRI) and computed tomography (CT) scans. He later developed atlantoaxial instability requiring surgery.

Case Description:

A 17-year-old male presented with neck pain, restricted cervical range of motion, but a normal neurological exam following a motor vehicle accident. The original X-rays showed a moderately displaced type-3 odontoid fracture. Additional MRI and CT scans excluded ligamentous injury, and he was initially treated with 13 weeks of halo vest immobilization. Radiographs 5 months later showed an enlarged atlanto-dens interval (e.g., >3 mm); the diagnosis of an odontoid fracture with an accompanying TAL injury was established, following which the patient successfully underwent a posterior C1–C2 fusion.

Conclusion:

Odontoid fractures require strict clinical and radiographic (X-ray, MRI, CT) surveillance to help rule out accompanying TAL injuries that may warrant surgical intervention.

Current Challenges in the Use of Computed Tomography and MR Imaging in Suspected Cervical Spine Trauma

There is controversy regarding the optimal imaging strategy in adult blunt trauma patients for suspected cervical spine trauma. Some investigators recommend negative computed tomography (CT) alone to clear the cervical spine in adult blunt trauma patients, while others insist that MR imaging is necessary, especially among obtunded adult blunt trauma patients. CT is an excellent imaging modality for bony cervical spine injury; however, there is a nonzero rate of clinically significant cervical spine injuries missed on CT. MR imaging has high sensitivity for soft tissue cervical spine injuries, but low specificity for the rare isolated unstable ligamentous cervical spine injury.

Management of Isolated Atlas Fractures: A Retrospective Study of 65 Patients

BACKGROUND

Jefferson fractures, or burst fractures of the C1 vertebra, can be managed surgically or conservatively, depending on their stability.

METHODS

We identified all patients who were treated for a C1 fracture at our institution between 1999 and 2016 for retrospective analysis. Patients with any other concurrent cervical fractures or nontraumatic etiology of fracture were excluded. Stability was defined as either lateral mass displacement ≥7 mm on computed tomography or presence of transverse atlantal ligament disruption on magnetic resonance imaging. We collected data on patients' demographic, clinical, and radiographic presentation and identified variables independently associated with instability at presentation and failure to achieve fusion at follow-up.

RESULTS

We identified 65 patients. On multivariable regression, instability at presentation was independently associated with atlantodens interval (odds ratio [OR] 2.357, 95% confidence interval [CI] [0.0629-1.271], P = 0.099) and type 3 fracture (OR 6.081, 95% CI [1.068-34.612], P = 0.042). Failure to achieve fusion was independently associated with age (OR 1.226, 95% CI [1.007-1.495], P = 0.043), motor vehicle collision as mechanism of injury (OR 22834.3, 95% CI [3.135-1.66e8], P = 0.027), and type 2 fracture (OR 168.537, 95% CI [1.743-16292.92], P = 0.028). Type 3 fracture was positively associated with halo vest for management (OR 17.171, 95% CI [2.882-102.289], P = 0.002) and negatively associated with a rigid cervical collar for management (OR 0.0616, 95% CI [0.0104-0.3653], P = 0.002). All 4 patients who underwent surgery presented with unstable fracture (P = 0.0187).

CONCLUSIONS

Atlantodens interval, mechanism of injury, and fracture type affect Jefferson fracture management decisions and outcomes, including instability at presentation and fusion at follow-up. Most fractures were managed nonsurgically regardless of stability.

Dynamic in vivo 3D atlantoaxial spine kinematics during upright rotation

Diagnosing dysfunctional atlantoaxial motion is challenging given limitations of current diagnostic imaging techniques. Three-dimensional imaging during upright functional motion may be useful in identifying dynamic instability not apparent on static imaging. Abnormal atlantoaxial motion has been linked to numerous pathologies including whiplash, cervicogenic headaches, C2 fractures, and rheumatoid arthritis. However, normal C1/C2 rotational kinematics under dynamic physiologic loading have not been previously reported owing to imaging difficulties. The objective of this study was to determine dynamic three-dimensional in vivo C1/C2 kinematics during upright axial rotation. Twenty young healthy adults performed full head rotation while seated within a biplane X-ray system while radiographs were collected at 30 images per second. Six degree-of-freedom kinematics were determined for C1 and C2 via a validated volumetric model-based tracking process. The maximum global head rotation (to one side) was 73.6±8.3°, whereas maximum C1 rotation relative to C2 was 36.8±6.7°. The relationship between C1/C2 rotation and head rotation was linear through midrange motion (±20° head rotation from neutral) in a nearly 1:1 ratio. Coupled rotation between C1 and C2 included 4.5±3.1° of flexion and 6.4±8.2° of extension, and 9.8±3.8° of contralateral bending. Translational motion of C1 relative to C2 was 7.8±1.5mm ipsilaterally, 2.2±1.2mm inferiorly, and 3.3±1.0mm posteriorly. We believe this is the first study describing 3D dynamic atlantoaxial kinematics under true physiologic conditions in healthy subjects. C1/C2 rotation accounts for approximately half of total head axial rotation. Additionally, C1 undergoes coupled flexion/extension and contralateral bending, in addition to inferior, lateral and posterior translation.