Biomechanical comparison of occipitoatlantal screw fixation techniques

2008 ◽  
Vol 8 (2) ◽  
pp. 143-152 ◽  
Author(s):  
Nicholas C. Bambakidis ◽  
Iman Feiz-Erfan ◽  
Eric M. Horn ◽  
L. Fernando Gonzalez ◽  
Seungwon Baek ◽  
...  
Keyword(s):  
Screw Fixation ◽  
Axial Rotation ◽  
Segmental Motion ◽  
Lateral Mass ◽  
Lateral Bending ◽  
Flexion Extension ◽  
Improved Stability ◽  
Fixation Techniques ◽  
Structural Graft ◽  
Lateral Mass Screws

Object The stability provided by 3 occipitoatlantal fixation techniques (occiput [Oc]–C1 transarticular screws, occipital keel screws rigidly interconnected with C-1 lateral mass screws, and suboccipital/sublaminar wired contoured rod) were compared. Methods Seven human cadaveric specimens received transarticular screws and 7 received occipital keel–C1 lateral mass screws. All specimens later underwent contoured rod fixation. All conditions were studied with and without placement of a structural graft wired between the skull base and C-1 lamina. Specimens were loaded quasistatically using pure moments to induce flexion, extension, lateral bending, and axial rotation while recording segmental motion optoelectronically. Flexibility was measured immediately postoperatively and after 10,000 cycles of fatigue. Results Application of Oc–C1 transarticular screws, with a wired graft, reduced the mean range of motion (ROM) to 3% of normal. Occipital keel–C1 lateral mass screws (also with graft) offered less stability than transarticular screws during extension and lateral bending (p < 0.02), reducing ROM to 17% of normal. The wired contoured rod reduced motion to 31% of normal, providing significantly less stability than either screw fixation technique. Fatigue increased motion in constructs fitted with transarticular screws, keel screws/lateral mass screw constructs, and contoured wired rods, by means of 19, 5, and 26%, respectively. In all constructs, adding a structural graft significantly improved stability, but the extent depended on the loading direction. Conclusions Assuming the presence of mild C1–2 instability, Oc–C1 transarticular screws and occipital keel–C1 lateral mass screws are approximately equivalent in performance for occipitoatlantal stabilization in promoting fusion. A posteriorly wired contoured rod is less likely to provide a good fusion environment because of less stabilizing potential and a greater likelihood of loosening with fatigue.

Biomechanical Comparison of Four C1 to C2 Rigid Fixative Techniques: Anterior Transarticular, Posterior Transarticular, C1 to C2 Pedicle, and C1 to C2 Intralaminar Screws

Neurosurgery ◽  
2006 ◽  
Vol 58 (3) ◽  
pp. 516-521 ◽  
Author(s):  
Samir B. Lapsiwala ◽  
Paul A. Anderson ◽  
Ashish Oza ◽  
Daniel K. Resnick
Keyword(s):  
Pedicle Screw ◽  
Screw Fixation ◽  
Axial Rotation ◽  
Lateral Bending ◽  
Transarticular Screw ◽  
Transarticular Screw Fixation ◽  
Flexion Extension ◽  
Fixation Techniques ◽  
Biomechanical Comparison ◽  
Cable Fixation

Abstract OBJECTIVE: We performed a biomechanical comparison of several C1 to C2 fixation techniques including crossed laminar (intralaminar) screw fixation, anterior C1 to C2 transarticular screw fixation, C1 to 2 pedicle screw fixation, and posterior C1 to C2 transarticular screw fixation. METHODS: Eight cadaveric cervical spines were tested intact and after dens fracture. Four different C1 to C2 screw fixation techniques were tested. Posterior transarticular and pedicle screw constructs were tested twice, once with supplemental sublaminar cables and once without cables. The specimens were tested in three modes of loading: flexion-extension, lateral bending, and axial rotation. All tests were performed in load and torque control. Pure bending moments of 2 nm were applied in flexion-extension and lateral bending, whereas a 1 nm moment was applied in axial rotation. Linear displacements were recorded from extensometers rigidly affixed to the C1 and C2 vertebrae. Linear displacements were reduced to angular displacements using trigonometry. RESULTS: Adding cable fixation results in a stiffer construct for posterior transarticular screws. The addition of cables did not affect the stiffness of C1 to C2 pedicle screw constructs. There were no significant differences in stiffness between anterior and posterior transarticular screw techniques, unless cable fixation was added to the posterior construct. All three posterior screw constructs with supplemental cable fixation provide equal stiffness with regard to flexion-extension and axial rotation. C1 lateral mass-C2 intralaminar screw fixation restored resistance to lateral bending but not to the same degree as the other screw fixation techniques. CONCLUSION: All four screw fixation techniques limit motion at the C1 to 2 articulation. The addition of cable fixation improves resistance to flexion and extension for posterior transarticular screw fixation.

scholarly journals Biomechanical Comparison of Lumbar Motion Unit Stability Following Posterior Instrumentation with Facet Spacers and Facet Screws

2019 ◽  
Vol 40 (2) ◽  
pp. 220-229
Author(s):  
Mu-Yi Liu ◽  
Tsung-Ting Tsai ◽  
Lih-Huei Chen ◽  
Wen-Huang Liang ◽  
Po-Liang Lai ◽  
...  
Keyword(s):  
Screw Fixation ◽  
Posterior Instrumentation ◽  
Testing Machine ◽  
Axial Rotation ◽  
Lateral Bending ◽  
Spacer Group ◽  
Flexion Extension ◽  
Fixation Techniques ◽  
Screw Group ◽  
Lumbar Motion

Abstract Purpose Lumbar posterior instrumentation for facet stabilization has become popular for the treatment of lumbar instability. The present study investigated and compared facet stabilization following lumbar posterior instrumentation with facet spacers and facet screws using porcine lumbar spines. Methods Eighteen L5–L6 lumbar motion units (LMUs) of the porcine spines were randomly divided into three groups (un-instrumented, facet-spacer and facet-screw). In the un-instrumented group (control), all ligamentous structures were preserved. In the facet-spacer group, two facet spacers were inserted into the joint spaces of the bilateral upper and lower facets. In the facet-screw group, two cannulated screws were used to transfix the bilateral upper and lower facets. With the use of a material testing machine, a gradually increasing moment of up to 6000 N-mm was generated in flexion, extension, lateral bending and torsion motions to compare facet stabilization among the groups. Results The facet-spacer group was significantly stiffer than the facet-screw group in extension (p = 0.013), whereas the facet-screw group was significantly stiffer than the facet-spacer group in axial rotation (p = 0.004). No statistically significant differences were observed between the two fixation techniques in flexion (p = 0.284) and lateral bending (p = 0.085). Conclusion Both facet-spacer and facet-screw fixation techniques significantly improve stability in a single LMU. Facet-spacer fixation provided better stabilization in extension, while facet-screw fixation provided better stabilization in axial rotation.

Biomechanical comparison of two new atlantoaxial fixation techniques with C1–2 transarticular screw–graft fixation

2006 ◽  
Vol 5 (4) ◽  
pp. 336-342 ◽  
Author(s):  
Roger Härtl ◽  
Robert H. Chamberlain ◽  
Mary S. Fifield ◽  
Dean Chou ◽  
Volker K. H. Sonntag ◽  
...  
Keyword(s):  
Gold Standard ◽  
Screw Fixation ◽  
Axial Rotation ◽  
Lateral Mass ◽  
Lateral Bending ◽  
Transarticular Screw ◽  
New Techniques ◽  
Transarticular Screw Fixation ◽  
Atlantoaxial Fixation ◽  
Lateral Mass Screws

Object Two new techniques for atlantoaxial fixation have been recently described. In one technique, C-2 intra-laminar screws are connected with C-1 lateral mass screws; in the second, C-1 and C-3 lateral mass screws are interconnected and C-2 is wired sublaminarly. Both techniques include a C1–2 interspinous graft. The authors compared these techniques with the gold-standard, interspinous graft–augmented C1–2 transarticular screw fixation and with a control C1–2 interspinous graft fixation procedure alone. Methods In six human cadaveric occiput–C4 specimens, nonconstraining 1.5-Nm pure moments were applied to induce flexion, extension, lateral bending, and axial rotation during which three-dimensional angular motion was measured optoelectronically. Each specimen was tested in the normal state, with graft alone (after odontoidectomy), and then in varying order after applying each construct with a rewired graft. All three constructs allowed significantly less angular motion at the C1–2 junction than the wired interspinous graft alone during lateral bending and axial rotation (p < 0.01, paired Student t-test) but not during flexion or extension. Transarticular screw fixation with an interspinous graft allowed less motion at the atlantoaxial junction than the two new constructs in several conditions. Differences were greater between the transarticular screw construct and the intralaminar screw construct than between the transarticular screw construct and the C1–3 lateral mass screw construct. During lateral bending and axial rotation, the C1–3 construct allowed less motion at the atlantoaxial junction than the intralaminar screw construct. Conclusions Biomechanically, the gold-standard C1–2 transarticular screw fixation outperformed the two new techniques during lateral bending and axial rotation. Wiring C-2 to C1–3 rods provided greater stability than C1–2 laminar screws, but it sacrificed C2–3 mobility. It is unknown whether the small differences observed biomechanically would lead to clinically relevant differences in fusion rates.

scholarly journals Single-Level In Vitro Kinematic Comparison of Novel Inline Cervical Interbody Devices With Intervertebral Screw, Anchor, or Blade

2019 ◽  
Vol 9 (7) ◽  
pp. 697-707 ◽  
Author(s):  
Paul M. Arnold ◽  
Ivan Cheng ◽  
Jonathan A. Harris ◽  
Mir M. Hussain ◽  
Chengmin Zhang ◽  
...  
Keyword(s):  
Axial Rotation ◽  
Biomechanical Study ◽  
Posterior Fixation ◽  
Lateral Mass ◽  
Flexion Extension ◽  
Before And After ◽  
Fixation Techniques ◽  
Lateral Mass Screws ◽  
Loading Modes

Study Design: In vitro cadaveric biomechanical study. Objective: To compare the biomechanics of integrated anchor and blade versus traditional screw fixation techniques for interbody fusion. Methods: Fifteen cadaveric cervical spines were divided into 3 equal groups (n = 5). Each spine was tested: intact, after discectomy (simulating an injury model), interbody spacer alone (S), integrated interbody spacer (iSA), and integrated spacer with lateral mass screw and rod fixation (LMS+iS). Each treatment group included integrated spacers with either screw, anchor, or blade integrated spacers. Constructs were tested in flexion-extension (FE), lateral bending (LB), and axial rotation (AR) under pure moments (±1.5 N m). Results: Across all 3 planes, the following range of motion trend was observed: Injured > Intact > S > iSA > LMS+iS. In FE and LB, integrated anchor and blade significantly decreased motion compared with intact and injured conditions, before and after supplemental posterior fixation ( P < .05). Comparing tested devices revealed biomechanical equivalence between screw, anchor, and blade fixation methods in all loading modes ( P > .05). Conclusion: All integrated interbody devices reduced intact and injured motion; lateral mass screws and rods further stabilized the single motion segment. Comparing screw, anchor, or bladed integrated anterior cervical discectomy and fusion spacers revealed no significant differences.

The cervical end of an occipitocervical fusion: a biomechanical evaluation of 3 constructs

2008 ◽  
Vol 9 (3) ◽  
pp. 296-300 ◽  
Author(s):  
Michael A. Finn ◽  
Daniel R. Fassett ◽  
Todd D. Mccall ◽  
Randy Clark ◽  
Andrew T. Dailey ◽  
...  
Keyword(s):  
Tracking System ◽  
Plate Fixation ◽  
Axial Rotation ◽  
Lateral Mass ◽  
Lateral Bending ◽  
Cross Link ◽  
3D Tracking ◽  
Flexion Extension ◽  
Biomechanical Evaluation ◽  
Rotation Motion

Object Stabilization with rigid screw/rod fixation is the treatment of choice for craniocervical disorders requiring operative stabilization. The authors compare the relative immediate stiffness for occipital plate fixation in concordance with transarticular screw fixation (TASF), C-1 lateral mass and C-2 pars screw (C1L-C2P), and C-1 lateral mass and C-2 laminar screw (C1L-C2L) constructs, with and without a cross-link. Methods Ten intact human cadaveric spines (Oc–C4) were prepared and mounted in a 7-axis spine simulator. Each specimen was precycled and then tested in the intact state for flexion/extension, lateral bending, and axial rotation. Motion was tracked using the OptoTRAK 3D tracking system. The specimens were then destabilized and instrumented with an occipital plate and TASF. The spine was tested with and without the addition of a cross-link. The C1L-C2P and C1L-C2L constructs were similarly tested. Results All constructs demonstrated a significant increase in stiffness after instrumentation. The C1L-C2P construct was equivalent to the TASF in all moments. The C1L-C2L was significantly weaker than the C1L-C2P construct in all moments and significantly weaker than the TASF in lateral bending. The addition of a cross-link made no difference in the stiffness of any construct. Conclusions All constructs provide significant immediate stability in the destabilized occipitocervical junction. Although the C1L-C2P construct performed best overall, the TASF was similar, and either one can be recommended. Decreased stiffness of the C1L-C2L construct might affect the success of clinical fusion. This construct should be reserved for cases in which anatomy precludes the use of the other two.

scholarly journals Biomechanical Evaluation of Unilateral Versus Bilateral C1 Lateral Mass-C2 Intralaminar Fixation

2017 ◽  
Vol 7 (3) ◽  
pp. 239-245 ◽  
Author(s):  
Nitin Bhatia ◽  
Asheen Rama ◽  
Brandon Sievers ◽  
Ryan Quigley ◽  
Michelle H. McGarry ◽  
...  
Keyword(s):  
Range Of Motion ◽  
Axial Rotation ◽  
Biomechanical Properties ◽  
Lateral Mass ◽  
Intact State ◽  
Flexion Extension ◽  
Fixation Techniques ◽  
Rotation Stiffness ◽  
Biomechanical Evaluation ◽  
C2 Fixation

Study Design: Biomechanical, cadaveric study. Objectives: To compare the relative stiffness of unilateral C1 lateral mass-C2 intralaminar fixation to intact specimens and bilateral C1 lateral mass-C2 intralaminar constructs. Methods: The biomechanical integrity of a unilateral C1 lateral mass-C2 intralaminar screw construct was compared to intact specimens and bilateral C1 lateral mass-C2 intralaminar screw constructs. Five human cadaveric specimens were used. Range of motion and stiffness were tested to determine the stiffness of the constructs. Results: Unilateral fixation significantly decreased flexion/extension range of motion compared to intact ( P < .001) but did not significantly affect axial rotation ( P = .3) or bending range of motion ( P = .3). There was a significant decrease in stiffness in extension for both unilateral and bilateral fixation techniques compared to intact ( P = .04 and P = .03, respectively). There was also a significant decrease in stiffness for ipsilateral rotation for the unilateral construct compared to intact ( P = .007) whereas the bilateral construct significantly increased ipsilateral rotation stiffness compared to both intact and unilateral fixation ( P < .001). Conclusion: Bilateral constructs did show improved biomechanical properties compared to the unilateral constructs. However, unilateral C1-C2 fixation using a C1 lateral mass and C2 intralaminar screw-rod construct decreased range of motion and improved stiffness compared to the intact state with the exception of extension and ipsilateral rotation. Hence, a unilateral construct may be acceptable in clinical situations in which bilateral fixation is not possible, but an external orthosis may be necessary to achieve a fusion.

Biomechanical evaluation of a simulated T-9 burst fracture of the thoracic spine with an intact rib cage

2014 ◽  
Vol 21 (3) ◽  
pp. 481-488 ◽  
Author(s):  
Tiffany G. Perry ◽  
Prasath Mageswaran ◽  
Robb W. Colbrunn ◽  
Tara F. Bonner ◽  
Todd Francis ◽  
...  
Keyword(s):  
Vertebral Body ◽  
Posterior Instrumentation ◽  
Burst Fracture ◽  
Axial Rotation ◽  
Segmental Motion ◽  
Lateral Bending ◽  
Rib Cage ◽  
Biomechanical Models ◽  
Flexion Extension ◽  
Biomechanical Evaluation

Object Classic biomechanical models have used thoracic spines disarticulated from the rib cage, but the biomechanical influence of the rib cage on fracture biomechanics has not been investigated. The well-accepted construct for stabilizing midthoracic fractures is posterior instrumentation 3 levels above and 2 levels below the injury. Short-segment fixation failure in thoracolumbar burst fractures has led to kyphosis and implant failure when anterior column support is lacking. Whether shorter constructs are viable in the midthoracic spine is a point of controversy. The objective of this study was the biomechanical evaluation of a burst fracture at T-9 with an intact rib cage using different fixation constructs for stabilizing the spine. Methods A total of 8 human cadaveric spines (C7–L1) with intact rib cages were used in this study. The range of motion (ROM) between T-8 and T-10 was the outcome measure. A robotic spine testing system was programmed to apply pure moment loads (± 5 Nm) in lateral bending, flexion-extension, and axial rotation to whole thoracic specimens. Intersegmental rotations were measured using an optoelectronic system. Flexibility tests were conducted on intact specimens, then sequentially after surgically induced fracture at T-9, and after each of 4 fixation construct patterns. The 4 construct patterns were sequentially tested in a nondestructive protocol, as follows: 1) 3 above/2 below (3A/2B); 2) 1 above/1 below (1A/1B); 3) 1 above/1 below with vertebral body augmentation (1A/1B w/VA); and 4) vertebral body augmentation with no posterior instrumentation (VA). A repeated-measures ANOVA was used to compare the segmental motion between T-8 and T-10 vertebrae. Results Mean ROM increased by 86%, 151%, and 31% after fracture in lateral bending, flexion-extension, and axial rotation, respectively. In lateral bending, there was significant reduction compared with intact controls for all 3 instrumented constructs: 3A/2B (−92%, p = 0.0004), 1A/1B (−63%, p = 0.0132), and 1A/1B w/VA (−66%, p = 0.0150). In flexion-extension, only the 3A/2B pattern showed a significant reduction (−90%, p = 0.011). In axial rotation, motion was significantly reduced for the 3 instrumented constructs: 3A/2B (−66%, p = 0.0001), 1A/1B (−53%, p = 0.0001), and 1A/1B w/VA (−51%, p = 0.0002). Between the 4 construct patterns, the 3 instrumented constructs (3A/2B, 1A/1B, and 1A/1B w/VA) showed comparable stability in all 3 motion planes. Conclusions This study showed no significant difference in the stability of the 3 instrumented constructs tested when the rib cage is intact. Fractures that might appear more grossly unstable when tested in the disarticulated spine may be bolstered by the ribs. This may affect the extent of segmental spinal instrumentation needed to restore stability in some spine injuries. While these initial findings suggest that shorter constructs may adequately stabilize the spine in this fracture model, further study is needed before these results can be extrapolated to clinical application.

Kinetic analysis of anterior cervical discectomy and fusion supplemented with transarticular facet screws

2014 ◽  
Vol 20 (5) ◽  
pp. 485-491 ◽  
Author(s):  
Vincent C. Traynelis ◽  
Jonathan Sherman ◽  
Eric Nottmeier ◽  
Vaneet Singh ◽  
Kirk McGilvray ◽  
...  
Keyword(s):  
Screw Fixation ◽  
Clinical Success ◽  
Axial Rotation ◽  
Anterior Cervical Discectomy ◽  
Screw Placement ◽  
Success Rates ◽  
Lateral Bending ◽  
Cervical Discectomy ◽  
Flexion Extension ◽  
Standard Plate

Object The clinical success rates of anterior cervical discectomy and fusion (ACDF) procedures are substantially reduced as more cervical levels are included in the fusion procedure. One method that has been proposed as an adjunctive technique for multilevel ACDF is the placement of screws across the facet joints (“transfacet screws”). However, the biomechanical stability imparted by transfacet screw placement (either unilaterally or bilaterally) has not been reported. Therefore, the purpose of this study was to determine the acute stability conferred by implementation of unilateral and bilateral transfacet screws to an ACDF construct. Methods Eight C2–T1 fresh-frozen human cadaveric spines (3 female and 5 male; mean age 50 years) were tested. Three different instrumentation variants were performed on cadaveric cervical spines across C4–7: 1) ACDF with an intervertebral spacer and standard plate/screw instrumentation; 2) ACDF with an intervertebral spacer and standard plate/screw instrumentation with unilateral facet screw placement; and 3) ACDF with an intervertebral spacer and standard plate/screw instrumentation with bilateral facet screw placement. Kinetic ranges of motion in flexion-extension, lateral bending, and axial rotation at 1.5 Nm were captured after each of these procedures and were statistically analyzed for significance. Results All 3 fixation scenarios produced statistically significant reductions (p < 0.05) in all 3 bending planes compared with the intact condition. The addition of a unilateral facet screw to the ACDF construct produced significant reductions at the C4–5 and C6–7 levels in lateral bending and axial rotation but not in flexion-extension motion. Bilateral facet screw fixation did not produce any statistically significant decreases in flexion-extension motion compared with unilateral facet screw fixation. However, in lateral bending, significant reductions at the C4–5 and C5–6 levels were observed with the addition of a second facet screw. The untreated, adjacent levels (C2–3, C3–4, and C7–1) did not demonstrate significant differences in range of motion. Conclusions The data demonstrated that adjunctive unilateral facet screw fixation to an ACDF construct provides significant gains in stability and should be considered a potential option for increasing the likelihood for obtaining a successful arthrodesis for multilevel ACDF procedures.

scholarly journals Biomechanical Comparison of Posterior Fixation Combinations with an Allograft Spacer between the Lateral Mass and Pedicle Screws

2020 ◽  
Vol 10 (20) ◽  
pp. 7291
Author(s):  
Soo-Bin Lee ◽  
Hwan-Mo Lee ◽  
Tae-Hyun Park ◽  
Sung Lee ◽  
Young-Woo Kwon ◽  
...  
Keyword(s):  
Pedicle Screws ◽  
Element Model ◽  
Axial Rotation ◽  
Posterior Fixation ◽  
Hybrid Technique ◽  
Lateral Mass ◽  
Lateral Bending ◽  
Flexion Extension ◽  
Hybrid Motion ◽  
Biomechanical Comparison

Background: There are a few biomechanical studies that describe posterior fixation methods with pedicle screws (PS) and lateral mass screws (LMS); the combination of both screw types and their effect on an allograft spacer in a surgically treated cervical segment is unknown. Methods: Finite element model (FEM) analyses were used to investigate the effects of a hybrid technique using posterior PS and LMS. Stress distribution and subsidence risk from a combination of screws under hybrid motion control conditions, including flexion, extension, axial rotation, and lateral bending, were investigated to evaluate the biomechanical characteristics of different six-screw combinations. Findings: The load sharing on the allograft spacer in flexion mode was highest in the LMS model (74.6%) and lowest in the PS model (35.1%). The likelihood of subsidence of allograft spacer on C6 was highest in the screws from the distal LMS (type 5) model during flexion and extension (4.902 MPa, 30.1% and 2.189 MPa, 13.4%). In lateral bending, the left unilateral LMS (type 4) model screws on C5 (3.726 MPa, 22.9%) and C6 (2.994 MPa, 18.4%) yielded the greatest subsidence risks, because the lateral bending forces were supported by the LMS. In counterclockwise axial rotation, the left unilateral LMS (type 4) model screws on C5 (3.092 MPa, 19.0%) and C6 (3.076 MPa, 18.9%) demonstrated the highest subsidence risks. Conclusion: The asymmetrical ipsilateral use of LMS and posterior PS in lateral bending and axial rotation demonstrated the lowest stability and greatest subsidence risk. We recommend bilateral symmetrical insertion of LMS or posterior PS and posterior PS on distal vertebrae for increased stability and reduced risk of allograft spacer subsidence.

Restabilization of the Occipitocervical Junction After a Complete Unilateral Condylectomy: A Biomechanical Comparison of Unilateral and Bilateral Fixation Techniques

2019 ◽  
Vol 19 (2) ◽  
pp. 157-164 ◽  
Author(s):  
Ilyas M Eli ◽  
Michael Karsy ◽  
Darrel S Brodke ◽  
Kent N Bachus ◽  
William T Couldwell ◽  
...  
Keyword(s):  
Biomechanical Testing ◽  
Axial Rotation ◽  
Neurological Injury ◽  
Occipital Condyle ◽  
Posterior Fixation ◽  
Lateral Bending ◽  
Flexion Extension ◽  
Fixation Techniques ◽  
The Difference ◽  
Biomechanical Comparison

Abstract BACKGROUND Occipitocervical instability may result from transcondylar resection of the occipital condyle. Initially, patients may be able to maintain a neutral alignment but severe occipitoatlantal subluxation may subsequently occur, with cranial settling, spinal cord kinking, and neurological injury. OBJECTIVE To evaluate the ability of posterior fixation constructs to prevent progression to severe deformity after radical unilateral condylectomy. METHODS Eight human cadaveric specimens (Oc-C2) underwent biomechanical testing to compare stiffness under physiological loads (1.5 N m). A complete unilateral condylectomy was performed to destabilize one Oc-C1 joint, and the contralateral joint was left intact. Unilateral Oc-C1 or Oc-C2 constructs on the resected side and bilateral Oc-C1 or Oc-C2 constructs were tested. RESULTS The bilateral Oc-C2 construct provided the greatest stiffness, but the difference was only statistically significant in certain planes of motion. The unilateral constructs had similar stiffness in lateral bending, but the unilateral Oc-C1 construct was less stiff in axial rotation and flexion-extension than the unilateral Oc-C2 construct. The bilateral Oc-C2 construct was stiffer than the unilateral Oc-C2 construct in axial rotation and lateral bending, but there was no difference between these constructs in flexion-extension. CONCLUSION Patients who undergo a complete unilateral condylectomy require close surveillance for occipitocervical instability. A bilateral Oc-C2 construct provides suitable biomechanical strength, which is superior to other constructs. A unilateral construct decreases abnormal motion but lacks the stiffness of a bilateral construct. However, given that most patients undergo a partial condylectomy and only a small proportion of patients develop instability, there may be scenarios in which a unilateral construct may be appropriate, such as for temporary internal stabilization.

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