Safe Zone For Screw Placement Research Articles

Cadaveric Examination of the Radiographic Safe Zone for Open Reduction and Internal Fixation of the Calcaneus Posterior Facet.

Background:The purpose of this study is to define a safe zone for screw placement on a lateral radiograph of the calcaneus taking into account the lateral to medial convexity of the posterior facet. Such findings may serve to improve surgical quality during open reduction and internal fixation (ORIF) of the posterior facet of the calcaneus.Methods:Eleven cadaveric calcanei were harvested and the articular margins of the posterior facet were outlined with a radiopaque wire. Lateral radiographs, similar to those used for intraoperative fluoroscopy, of each specimen were obtained and calibrated to a standardized marker. The proximal-to-distal length of the posterior facet was then divided into quadrants. The greatest height difference between the superolateral and inferomedial surfaces outlined by the radiopaque marker were measured in the 2 most posterior quadrants, as screw insertion in this area would be mostly likely to risk screw penetration during ORIF.Results:The average distance from the osseous surface to the radiographic marker was 3.3 ± 1.2 mm in the most posterior quadrant (fourth quadrant) and 3.2 ± 1.6 mm in the quadrant just anterior to this (third quadrant). The range for unsafe screw placement was 1.7 to 5.6 mm below the osseous surface in the fourth quadrant and 1.1 to 6.6 mm in the third quadrant.Conclusion:Intraoperative radiographic assessment of the safety of subchondral posterior facet screws does not correlate to its osteology. Because of the superolateral to inferomedial convexity of the posterior facet of the calcaneus, overly long screws may appear to be radiographically intraosseous, though in actuality the screw may be intra-articular. On average, screws placed in the fourth quadrant of the facet are at less risk if 3.3 mm inferior to the upper margin of the osseous shadow on fluoroscopic imaging and 3.2 mm inferior in the third quadrant. Though limited by a small sample size, this study sets a foundation for future research into this complex osteology.Level of Evidence:Level V, mechanism-based reasoning.

Is Sacral Dysmorphism Protective Against Spinopelvic Dissociation? Multicenter Case Series.

Investigate the incidence of sacral dysmorphism (SD) in patients with spinopelvic dissociation (SPD). Retrospective case series. Two academic level 1 trauma centers. One thousand eight hundred fifty adult patients with sacral and pelvic fractures (OTA/AO 61-A, B, C). Plain pelvic radiographs and CT scans. Incidence of SD in patients with SPD. Secondary radiographic evaluation of fracture classification and deformity on sagittal imaging. Eighty-two patients with SPD were identified, and 12.2% displayed features of SD, significantly less than reported in the literature. The S2 sacral body was the most common horizontal fracture location in patients with SD and nondysmorphic sacra (ND). Roy-Camille type I patterns were more common in ND (35%), versus type II in SD patients (40%). SD patients had lower body mass indexes (19.7 vs. 25.2, P = 0.001). Segmental kyphosis (22.5 degrees ND vs. 23.8 degrees SD, P = 0.838) and sacral kyphosis (26 degrees ND vs. 31 degrees SD, P = 0.605) were similar between groups. Percutaneous fixation was the most common surgical technique. We report a significantly lower prevalence of SD in patients with SPD than previously reported in the literature. This suggests that variations in sacral osseous anatomy alter force transmission across the sacrum during traumatic loading, which may be protective against certain high-energy fracture patterns. Preoperative evaluation of sacral anatomy is critical, not only in determining the size and orientation of sacral segment safe zones for screw placement, but also to better understand the pathomechanics involved in sacral trauma. Prognostic Level IV. See Instructions for Authors for a complete description of levels of evidence.

Safe Zones for Spinopelvic Screws in Patients With Lumbosacral Transitional Vertebra.

Retrospective matched-pair analysis. Lumbosacral transitional vertebrae (LSTV) have a reported prevalence of 4-36% in the population. The safe zones for screw placement for spinopelvic fusion in adult spinal deformity surgery for patients with LSTV have not been described in the literature. Our study aimed to assess the safety of S1-pedicle screw (S1PS), S2-alar screw (S2AS), S2-alar-iliac screw (S2AIS), and iliac screw (IS) placement in patients with LSTV. Out of the 819 examined patients, 49 patients with LSTV were included in our retrospective analysis with a matched pair control group. We used the 3-dimensional planning tool mediCAD for screw placement of S1PS, S2AS, S2AIS, IS with different angles, length and diameters. We evaluated a total of 10 192 screw trajectories. No serious complications occurred due to the trajectories used for S1PS. LSTV increased the risk of vessel injury for S2AS trajectories (P = .001) but not for S2AIS (P = .526). Besides the presence of an LSTV, the screw trajectory had a major influence on the frequency of serious complications. Sacral anchoring of long spinal constructions using S1PS, S2AS, S2AIS and IS is also possible in the presence of LSTV. For S2AS the trajectory with 30° lateral and caudal angulation of 10° showed the least vascular injuries and the least sacro-iliac-joint violations in patients with LSTV. S2AIS trajectories with 40° lateral and 0° sagittal angulation reduced the risk of serious complications in our patients collective with LSTV.

Is There a Safe Zone for Lateral Border Fixation of Mandibular Angle Fractures?

Cross-sectional study design. There are multiple accepted treatment options for internal fixation of mandibular angle fractures. The purpose of this study was to determine if there is a safe zone for lateral border fixation of mandibular angle fractures. One hundred coronal images of facial computed tomography (CT) scans were reviewed on patients between the ages of 18 to 48. Measurements were taken in the area of the second and third molar region related to the inferior border to the superior extent of the inferior alveolar canal and apex of the second molar root, along with buccal cortical measurements to the inferior alveolar canal and apical third of the second molar root. The average measurement of the inferior border in the second molar area to the inferior alveolar canal and apex of the root was 1.12 cm (0.70-1.77) and 1.39 cm (0.91-2.30), respectively. The average measurement of the inferior border of the third molar to the inferior alveolar canal was 1.26 cm (0.78-1.83). The average measurement of the buccal cortex of the second molar to the inferior alveolar canal and apical one-third of the root was 0.64 cm (0.34-1.25) and 0.59 cm (0.33-0.98), respectively. The average measurement of the third molar buccal cortex to the inferior alveolar canal was 0.45 cm (0.18-0.98). In the area of the second molar region, there is no ubiquitous safe zone for screw placement, cortical bone thickness is more critical than vertical placement of the fixation plate and screws. In the third molar region, cortical bone thickness and vertical orientation may provide a safe zone for screw placement.

Radiographic Quantification of Safe Zones for Screw Placement in the Femoral Neck: A Computed Tomography-Based Analysis.

The purpose of this study is to design a radiographic map of the femoral neck showing proportion-based locations of the safe zones for screw placement with widest bony extents in anteroposterior and lateral radiographs using normal computed tomography-based data. We analyzed computed tomography-based studies of 50 intact normal proximal femora equally from male and female subjects. Using software-developed radiographs, the proportionate locations of the maximal anteroposterior and cephalocaudal extents in both constricted zones were measured. The width of the femoral neck in the measurement zone was taken as the reference for calculation of proportions. For anteroposterior radiographs, the anteroposterior safe zones in the femoral neck are located at the gradients of 34.21% and 34.33% from the superior border in midcervical and basicervical regions, respectively. In lateral radiographs, they correlate with the visible anterior extent of femoral neck and lie at a gradient of 7.16% and 11.79% from the visible posterior border in midcervical and basicervical regions, respectively. In lateral radiographs, the calcar-based cephalocaudal safe zone was located at the gradients of 43.49% and 39.53% from the visible posterior border in midcervical and basicervical regions, respectively. In anteroposterior radiographs, cephalic limit of the calcar-based safe zone is located at the gradients of 9.63% and 17.82% from the superior border in midcervical and basicervical regions, respectively. Radiographic margins cannot be reliably trusted for screw fixation of femoral neck fractures. The proportionate locations of the anteroposterior and calcar-based cephalocaudal safe zones with widest bone stock in anteroposterior and lateral fluoroscopic projections can help in the safe placement of screws for fixation of femoral neck fractures.

Safe Zone for Insertion of a Fibular Lag Screw in Ankle Fracture Fixation

Category: Ankle Introduction/Purpose: Fibular lag screw placement during ankle fracture fixation is not without risk. The screw placement endangers either the tendons of the peronei or the posterior rim of the incisura if misplaced. Our aim was to identify a predictable safe zone for screw placement. Methods: 45 radiographs of Weber B fractures were reviewed to determine the typical height of lag screw entry and exit points. 63 MRI scans of anatomically normal ankles were reviewed to evaluate tendon position and syndesmosis location. The safe zone could then be determined using composite images. Results: On review of the 45 ankle fracture radiographs; the typical lag screw exit point was found to be 14.2 mm above the ankle joint (95% Confidence Interval: 11.3-17.1 mm). Using the composite MRI images, there was a consistent flat anterior aspect of the fibula at this level. A safe zone trajectory was seen to occur between 31 and 45 degrees taken from the anterior aspect of the flat fibular surface at this level. The minimum distance to at-risk structures using this trajectory was 4 mm. If this consistent entry point is used, the MRI scans demonstrated that if the drill was aimed towards the medial edge of the Achilles tendon, the correct trajectory would be performed. Conclusion: The flat surface of the fibula is a constant landmark on MRI and is visible during surgery. The peroneal tendons and posterior rim of the incisura have a constant predictable position related to this. The safe zone for insertion of a lag screw is between 31 and 45 degrees medial to the anterior aspect of this flat surface. This represents aiming the drill towards the medial aspect of the Achilles tendon.

Insertion of psoas minor tendon at pelvic brim, a novel anatomic landmark for extra-articular, screw placement through Stoppa approach

BackgroundThe psoas minor partially inserted to the superior pelvic brim. And the plate used to fix the acetabular fracture has always been positioned at the pelvic brim after reduction through the Stoppa approach. However, there are few studies depicting the clinical significance of the psoas minor. The purpose of this paper was to explore the relationship between the insertion of the psoas minor tendon at the pelvic brim (IPMTPB) and screw placement through the Stoppa approach. Materials and methodsFifteen cadavers were dissected for adequate exposure to IPMTPB in our study. However, not all specimens had a psoas minor. For the specimens with IPMTPB, the posterior and anterior edges were used as the first and second entry points, dividing the area from the sacroiliac joint to the pubic symphysis into three zones (d1, d2 and d3). The average proportion of each zone was obtained after measurement the three zones, to locate the two entry points for the specimens without a psoas minor. From the longitudinal Stoppa incision, the first wire was inserted horizontally, and the second wire was placed vertical to the bone surface. Fluoroscopy and computed tomography (CT) were conducted to examine the relationship between the wires and the acetabulum. ResultsThere was a psoas minor in sixteen hemipelvises (53.33%). After measurement and calculation, we determined that the average proportions of zones d1, d2, and d3 were 28.03%, 29.14%, and 42.83%, respectively. For all specimens, the wires were successfully inserted, and the trajectories of the wires were outside the hip joint cavity. ConclusionsIPMTPB could be used as an anatomic landmark of safe zones for screw placement through the Stoppa approach. For cases without a psoas minor, the zones for extra-articular screw placement could be determined through the measurements in this paper.

36 - Subaxial cervical pedicle screw fixation: safe zone for screw placement—a systematic review

BACKGROUND CONTEXT: The use of pedicle screw fixation in the cervical spine has been considered an unacceptable risk for intraoperative neurovascular (nerves and blood vessels) injuries due to the proximity of the cervical neurovascular structures to the pedicle.

C1 lateral mass screw placement via the posterior arch: a technique comparison and anatomic analysis

Background contextInstrumentation of C1 is becoming increasingly common. Starting points initially described for C1 lateral mass screws at the lateral mass/posterior arch junction are technically challenging. Recently, a number of techniques have evolved advocating varying starting points and trajectories. Despite being technically easier, there are new safety concerns. Insufficient evidence exists for optimal C1 lateral mass screw placement with starting points in the posterior arch. PurposeTo determine anatomic variability of the C1 lateral mass and posterior ring and to compare safety and feasibility of C1 lateral mass screw placement techniques via the posterior arch. Study designDescriptive anatomy for surgical technique. MethodsOne hundred thin-cut cervical spine computed tomography scans were acquired and formatted for virtual surgery. Four different described techniques were used for virtual placement of C1 lateral mass screws. Success was defined as avoidance of critical structures including the transverse foramen, vertebral groove, and spinal dura. Anatomic variability of the C1 vertebra and safe zones for screw placement were also clarified. ResultsOverall screw placement success for the four techniques was 50% (Resnick), 92% (Tan et al.), 58% (Ma et al.), and 85% (Christensen et al.). Average posterior arch height was 6.7±2.1 mm, and vertebral groove height 4.9±1.1 mm was the most limiting dimension to safe screw placement. A safe zone for screw placement was found in 100% of cases (200 screws), 17.0±1.1 mm from midline and a width of 12.6±1.7 mm. Posterior tubercle morphology was variable. ConclusionsC1 lateral mass screws could be virtually placed bilaterally in each of 100 clinical cases without violating critical structures. However, none of the previously described approaches worked in every case because of significant anatomic variability. The vertical starting point was particularly critical, and vertebral groove height was the most limiting variable. Although a reliable safe zone could be found in every case, preoperative planning is essential to avoid critical structures.

The ‘Safe Zone’ for Extra-Articular Screw Placement During Intra-Pelvic Acetabular Surgery

Intra-articular screw placement during acetabular surgery must be avoided. To date, no anatomic guidelines exist to prevent intra-articular screw placement in the treatment of complex acetabular fractures by the ilioinguinal or Stoppa approach. Ninety-three pelvic computed tomography scans were analyzed and the femoral head diameter (FHD) and the intersacroiliac joint distance (ISIJD) were referenced as anthropometric parameters. A safe zone for screw placement avoiding the acetabulum for specific anatomic landmarks such as the ischial spine, sciatic notch, and obturator canal was defined and correlated to the FHD and ISIJD. The mean FHD was 45.5 mm (standard deviation, 3.96). The mean ISIJD was 117.9 mm (standard deviation, 6.32). The correlation for the distance of the ischial spine in relation to the femoral head diameter was 0.59 (P = 0.0101) and for the ISIJD 0.274 (P = 0.0501). A safe zone of 28 mm from the ischial spine aiming laterally for a FHD greater than 47 mm and 23 mm for a FHD less than 47 mm could be identified. The safe zone for potential screw placement near the sciatic notch was 11 mm (correlation FHD 0.234, P = 0.0501; ISIJD 0.412, P = 0.0101). A safe zone of 5 mm in relative distance of the obturator canal could be determined for screws on the pubic ramus. This study identifies a safe zone for screw placement in the treatment of acetabular fractures when operated by an anterior approach. These findings have the potential to prevent further injury to the acetabular and femoral cartilage as a result of the surgical trauma and minimize intraoperative irradiation time.

Evaluation of anatomic landmarks and safe zones for screw placement in the atlas via the posterior arch

Several studies have evaluated quantitative anatomic data for direct lateral mass screw fixation. To analyze anatomic landmarks and safe zones for optimal screw placement through the posterior arc of the human atlas, morphometric parameters of 41 adult native human atlas specimens were quantitatively measured. Internal dimensions of the atlas (lateral mass, maximum and minimum intraosseous screw length), minimum height and width of the posterior arc and optimal screw insertion angles were defined on pQCT scans. By this, an optimal posterior screw insertion point (OIP) and a preferable screw direction (PSD) through the posterior arch into the lateral mass of C1 were defined. External dimensions (transverse and sagittal diameter) as well as the width of the mid-portion of C1 lateral mass were significantly higher in male specimens. The mean height of the posterior arch at the vertebral artery groove was 4.1 +/- 0.8 mm in female and 4.6 +/- 0.9 mm in male specimens. The optimal screw insertion point was located 21.6 +/- 1.7 mm in female and 23.6 +/- 2.3 mm in male lateral from the posterior tubercle of C1 (P < 0.01). The preferable screw direction was a mean medial inclination of 7.9 +/- 1.9 degrees in female and 7.3 +/- 2.7 degrees in male specimens and a mean rostral direction of 2.4 +/- 1.8 degrees in female and 3.1 +/- 1.7 degrees in male specimens. In conclusion, the presented study provides information for the use and design of upper cervical spine instrumentation techniques, such as screw placement to C1 via the posterior arch. The characterization of working areas and safe zones (OIP, PSD) might contribute to a minimization of screw malposition in this highly demanding instrumentation technique.

Acetabular Anatomy and Transacetabular Screw Fixation at the High Hip Center

A quadrant system that defines the safe acetabular locations for screw placement exists for the anatomic hip center. We wanted to develop a similar system for the high hip center. The purposes of our study were to identify the anatomic structures at risk during placement of transacetabular screws in the high hip center, to identify maximum bone depth for screw purchase, and to determine if a high hip center quadrant system could be validated to guide placement of screws during acetabular arthroplasty. For this cadaver study of nine pelves, an acetabulum was reamed superiorly into the high hip center a distance equal to (1/2) of the native acetabular diameter. Screws exiting the acetabular bone by 15 mm were inserted before a computed tomography scan and a precise anatomic dissection were done. Structures at risk of penetration by screws include the external iliac vessels, the obturator nerve and vessels, the superior gluteal nerve and vessels, and the sciatic nerve. We found that a quadrant system at the high hip center can demarcate safe zones for screw placement. At the high hip center, only the peripheral (1/2) of the posterior quadrants are safe for screw placement.

Anatomic placement of the Herbert-Whipple screw in scaphoid fractures: a cadaver study.

An anthropometric cadaver study was performed to determine the optimal placement of a Herbert-Whipple screw in the scaphoid. Twenty pairs of cadaver wrists were examined and anthropometric analysis of the scaphoids using plain radiography, computed tomography, and en bloc resection was performed. Scaphoid vascularity was evaluated using India ink injection. Radiographic and actual measurements of height and width confirmed symmetry in paired scaphoids. Anatomic evaluation defined a safe zone for screw placement. K-wire placement defined the extent of this zone. The entry point of this zone was found by moving dorsally and distally 15% of the lateral radiographic length of the contralateral scaphoid from the volar aspect of the scaphoid tuberosity. Moving 10% of this same length from the membranous central portion of the scapholunate ligament defined the target site. Computed tomography confirmed proper position of the screw to maximize bony purchase and avoid articular penetration while maintaining dorsal vascular integrity.