Monocortical Screws Research Articles

Point of View: Biomechanical Evaluation of a Newly Developed Monocortical Expansion Screw for Use in Anterior Internal Fixation of the Cervical Spine

Point of View: Biomechanical Evaluation of a Newly Developed Monocortical Expansion Screw for Use in Anterior Internal Fixation of the Cervical Spine

Comparison between the rigidity of bicortical screws and a miniplate for fixation of a mandibular setback after a simulated bilateral sagittal split osteotomy

Purpose: This investigation compared the biomechanical stability of three bicortical screws with that of a single four-hole miniplate after 5-mm mandibular setback after a bilateral sagittal split osteotomy (BSSO) in cadaver mandibles. Materials and Methods: Thirty human cadaver hemimandibles underwent BSSO followed by two different rigid fixation techniques. All specimens had no third molar, bony pathology, or evidence of mandibular fracture, and there was no history of renal disease or hyperparathyroidism. The specimens were randomly divided into two groups. In group I, three bicortical screws were placed at the superior border, and in group II, one four-hole miniplate was secured on the external oblique ridge with four monocortical screws. The bony height of the mandible was recorded. Maximum resistance load (MRL), the greatest load recorded just before a sudden decrease in load level (bone or fixation failure), was recorded when the mandibles were tested in a compression machine. Multiple regression analysis was used to evaluate the differences in bone height and the MRL between groups I and II. Results: The mean bone height in groups I and II were 28.64 ± 2.50 mm and 28.72 ± 4.08 mm, respectively. The mean MRL in group I (20.49 ± 7.22 kg) was greater than in group II (17.41 ± 7.81 kg). The multiple regression analysis showed no significant difference in the bone height and the MRL between group I and group II (beta = 2.3492, P = .4114). Conclusion: There was no statistically significant difference in stability provided the two techniques.

The tension wire method: a simple, effective means of mandibular fixation.

To investigate the use of a tension wire band secured to monocortical screws for open reduction and internal fixation of simple, displaced, and/or unstable mandibular fractures. Retrospective review with follow-up duration of at least 6 weeks. Level I university trauma center. Twenty-nine patients (27 males and 2 females), aged 7 to 46 years. Ten patients had unilateral fractures (1 patient had 3 separate unilateral fractures) and 19 had bilateral fractures; 34 of 50 fractures were displaced; 19 were open intraorally. The location of fractures and the number (number repaired) were as follows: symphysis or parasymphysis, 13 (10); body, 9 (8); angle, 18 (14); ramus, 6 (4); and subcondylar, 4 (0). Two fractures were each comminuted into 3 fragments, and 1 patient had unilateral parasymphysial, body, and ramus fractures. Intermaxillary fixation was done in all patients except 1 child. Intraoral approaches were used exclusively. A pair of monocortical 2.0-mm screws were placed perpendicular to fracture lines, with 24-gauge wire loops passed around the screws and tightened to bring the fracture into reduction and provide stable fixation. A percutaneous trocar system was used to insert screws at the body, angle, and ramus sites. There were no instances of infection, malunion, or malocclusion in the 33 fractures repaired with this technique. A typical intermaxillary fixation with open reduction and internal fixation of a posterior fracture was done in less than 2 hours. Open reduction and internal fixation of simple mandibular fractures with tension wire bands around monocortical screws is a simple, quick, and effective technique.

An in vitro comparison of the mechanical characteristics of three sagittal ramus osteotomy fixation techniques

Purpose: The purpose of this in vitro investigation was to compare the mechanical characteristics of three techniques for the rigid internal fixation of simulated sagittal ramus osteotomies in terms of their ability to resist vertical loads resembling masticatory forces. Materials and Methods: Fifteen bovine ribs of similar size and shape were subjected to uniform osteotomies resembling a sagittal ramus osteotomy. They were divided into three groups and fixed with three 2.0-mm bicortical positional screws in an inverted L-pattern, eight monocortical screws and a sagittal strut plate, or a hybrid system using three 2.0-mm bicortical positional screws, five 2.0-mm monocortical screws, and a sagittal strut plate. The repaired ribs were secured in a jig, subjected to vertically deforming forces, and evaluated for yield load, yield point, maximum load, displacement at maximum load, load at 3.0 mm displacement, and stiffness. The various groups were compared statistically with a Scheffe multiple comparison test. Results: No statistically significant differences existed between the positional screw group and the hybrid group ( P > .05). No statistically significant differences were noted in yield displacement, yield load, displacement at maximum load, for any of the groups ( P > .05). There were differences noted between the monocortical strut group and other groups in maximum load, load at 3.0 mm displacement, and stiffness ( P > .05). Conclusions: The monocortically fixed sagittal strut plate offers less resistance to maximum load and loads at 3.0 mm displacement and is less stiff than either a three-positional screw system or a hybrid system. No differences existed between the three positional screw systems and the hybrid system. However, given the low yield loads and yield displacements, these differences may not be clinically important.

A comparison of mandibular angle fracture plating techniques

Objective The purpose of this investigation was to compare the conventional technique of mandibular angle fracture plating with two biomechanically dissimilar techniques in their abilities to resist vertical loads similar to masticatory forces. Study design Three groups of five synthetic hemimandibles with simulated fracture repairs were compared for their capabilities to resist vertical deformation. The conventional group was stabilized with a thin tension band system at the superior border and thick stabilization plate system at the inferior border. The nontraditional group was stabilized with a thick tension band system at the superior border and thin stabilization plate system at the inferior border. The two miniplate group was stabilized with a thin tension band system at the superior border and thin stabilization plate at the inferior border. A cantilever beam design was used. Testing was performed with an Instron 8511.20 (Instron Corp., Canton, Mass.) mechanical testing device. The three groups were compared with a two way analysis of variance. Results The forces resisted by the conventional group (167.6±18.2 N), the nontraditional group (156.3±33.9 N), and two miniplate group (154.0±18.4 N) were not statistically different (F=0.44, p>0.66). All failures occurred at the tension bands secured with monocortical screws. Conclusions Under the conditions described in this in vitro investigation, plate thickness or pattern made no difference. All failures in this experiment occurred with monocortical screws in the superior border tension band system.

Stabilization of the short sagittal split osteotomy: in vitro testing of different plate and screw configurations

Using a 3-dimensional in vitro model, the stability of different types of osteosynthesis for the short sagittal osteotomy was tested. The following four groups of plate and screw configurations were evaluated: Group I: Fixation with miniplates using monocortical screws only, Group II: Fixation with miniplates, but with two of the screws engaging both fragments, Group III: Same as group II with an additional position screw, Group IV: Fixation with 3 position screws. The stability obtained with miniplate fixation using monocortical screws only (group I) was by a factor of 2.9 less than position screw fixation (group IV), which is considered to be an approved standard. In order to increase the stability of miniplate fixation, the screws in the area of overlapping bone should engage both fragments.

The biomechanics of the PC-Fix internal fixator

Conventional fracture treatment implants, i.e. bone plates and intramedullary nails, but also a great variety of special devices, rely on a mixed mode of load transfer between the bone segments and the implant. The bending moments and transverse forces are usually balanced by reactive forces created by compression in the areas of contact, while the torsional and the axial forces are most frequently transferred by bone screws, directly, or by screw-dependent friction between the implant and the bone. In all cases, contact between the implant and the bone is required and is achieved either by careful adaptation of the implant to the bone or by chance fit. Vascular damage in these areas of direct contact has been shown to account for most of the implant-related damage to the bone. The Point Contact Fixator (PC-Fix: an internal plate and screw fixation system, the function of which is based on similar mechanical principles to the external fixator) is a general purpose internal fixation system in which the mixed mode of load transfer has been eliminated in favour of screw-only transfer, made possible by locking of the screw head into the “plate” of the PC-Fix. The possibility to contour the implant and to use the screws without any special preparatory steps gives the PC-Fix the versatility of conventional plating systems, while practically eliminating their major drawback of direct contact to the bone. Locking of the screw head in the PC-Fix permits transfer of the moments between the screw and the implant. This has allowed a further reduction of damage to the bone by the use of monocortical screws. A theoretical comparison of the mechanisms of load transfer between the Point Contact Fixator and the Dynamic Compression Plate has been demonstrated by in vitro experimental data. The bone-implant construct is generally stronger in the case of the internal fixator provided the bone treated can withstand implant yielding loads. The mechanical conditions at the fracture site for sub-yield levels of loading are dominated by the technique of application and are expected to be comparable for the two implants. It appears that most of the advantages of the PC-Fix observed to date in in vivo experimental fracture treatment can be explained by the reduction of implant-related damage to the bone without any compromise on the mechanical function of fracture stabilization.

Basic concepts relevant to the design and development of the point Contact Fixator (PC-Fix)

The PC-Fix represents a novel approach to internal fixation. It has been designed for the optimal preservation of biology, to enhance fracture healing, and to improve resistance to infection. The PC-Fix can be understood functionally as a completely implanted external fixator. Although the PC-Fix resembles a plate, its biological and mechanical characteristics are different. The implant has minimal contact to the bone. Instead of pressing the splint onto the bone to produce friction, the atraumatic monocortical screws act as pegs connecting the splint to the bone. Force transmission by friction is abolished, therefore, the contact to the bone can be minimized. Ease of application is enhanced by a specially designed distractor. The function of the distractor is based on hinges which permit and maintain the reduction of the fracture fragments while the axes are aligned and rotation adjusted.

The biomechanics of the PC-Fix internal fixator

Conventional fracture treatment implants, i.e. bone plates and intramedullary nails, but also a great variety of special devices, rely on a mixed mode of load transfer between the bone segments and the implant. The bending moments and transverse forces are usually balanced by reactive forces created by compression in the areas of contact, while the torsional and the axial forces are most frequently transferred by bone screws, directly, or by screw-dependent friction between the implant and the bone. In all cases, contact between the implant and the bone is required and is achieved either by careful adaptation of the implant to the bone or by chance fit. Vascular damage in these areas of direct contact has been shown to account for most of the implant-related damage to the bone. The Point Contact Fixator (PC-Fix: an internal plate and screw fixation system, the function of which is based on similar mechanical principles to the external fixator) is a general purpose internal fixation system in which the mixed mode of load transfer has been eliminated in favour of screw-only transfer, made possible by locking of the screw head into the “plate” of the PC-Fix. The possibility to contour the implant and to use the screws without any special preparatory steps gives the PC-Fix the versatility of conventional plating systems, while practically eliminating their major drawback of direct contact to the bone. Locking of the screw head in the PC-Fix permits transfer of the moments between the screw and the implant. This has allowed a further reduction of damage to the bone by the use of monocortical screws. A theoretical comparison of the mechanisms of load transfer between the Point Contact Fixator and the Dynamic Compression Plate has been demonstrated by in vitro experimental data. The bone-implant construct is generally stronger in the case of the internal fixator provided the bone treated can withstand implant yielding loads. The mechanical conditions at the fracture site for sub-yield levels of loading are dominated by the technique of application and are expected to be comparable for the two implants. It appears that most of the advantages of the PC-Fix observed to date in in vivo experimental fracture treatment can be explained by the reduction of implant-related damage to the bone without any compromise on the mechanical function of fracture stabilization.

Sagittal split advancement osteotomies stabilized with miniplates. A 2–5-year follow-up

Skeletal stability, temporomandibular joint (TMJ)-function, and inferior alveolar nerve function were evaluated in 103 patients with mandibular hypoplasia who were treated with bilateral sagittal split osteotomies to advance the mandible. Stable internal fixation was obtained with miniplates and four monocortical screws. The follow-up period was at least 24 months (average: 32 months; maximum: 60 months). The average B-point advancement was 5.85 mm. Ninety-three patients of 103 (90.3%) had no appreciable relapse at B-point. Eight (7.7%) patients had relapse because of condylar resorption. Maximum mouth opening decreased an average of 1 mm. Sixty-eight percent of the patients with preoperative TMJ-dysfunction symptoms reported improvement or resolution of their symptoms. Sixteen percent experienced worsening of their TMJ symptoms. Permanent neurosensory deficits were identified in five patients (five sides).

A comparative in vitro study on fixation of sagittal split osteotomies with Würzburg ® screws, Champy ® miniplates, and Biofix ® (biodegradable) rods

A bilateral sagittal split osteotomy was performed on seven fresh cadaver mandibles. Three different systems of fixation were mechanically tested on 14 sites. Tensile diagrams were obtained in which the (offset-) yield point was measured. This resulted in mean yield stresses of 199 N for bicortical selftapping screws ( n = 6), 49 N for miniplates with monocortical screws ( n = 5), and 113 N for bicortical biodegradable rods ( n = 3).

Management of malunited mandibular condylar fractures

The non-surgical treatment of mandibular condylar fractures, may occasionally result in articular imbalance and temporomandibular joint dysfunction. This may be attributed to condylar head displacement and resorption, resulting in a shortened vertical ramus and lost posterior vertical facial height. Restoring the vertical ramus height is essential in the treatment of such dysfunction, and may be accomplished by unilateral, or bilateral ramus osteotomies. Four examples of patients treated with mandibular ramus osteotomies to restore vertical ramus height, with subsequent improvement in occlusal balance and function are presented. The use of the sagittal split mandibular osteotomy and the external vertical ramus osteotomy, stabilized with small osseous plates, and monocortical screws, is discussed.

Skeletal stability following sagittal split osteotomy using monocortical miniplate internal fixation

Skeletal stability was evaluated in 20 patients with mandibular hypoplasia, treated with bilateral sagittal split osteotomies to advance the mandible. Stable internal fixation was obtained using osseous miniplates and monocortical screws. Intermaxillary fixation was released after 5.15 days (range 1 to 11 days). The average B-point advancement was 6.07 mm (range 2.25 to 17.5 mm) and the average Pogonion-point advancement was 5.39 mm (range 1.75 to 14 mm). Mandibular range of motion, TMJ dysfunction and neurosensory deficits were also evaluated. The follow-up period averaged 8.85 months (range 6 to 14 months) and final evaluations were made after completion of orthodontics. Relapse measured at B-point was 10.7% and at Pogonion was 18.7%. Maximal opening decreased an average of 0.47 mm. Symptoms in 8 patients with TMJ dysfunction resolved, while 3 others developed TMJ dysfunction following surgery. Neurosensory deficits were subjectively identified in 9 patients (10 sides) and were objectively measured in 5 patients (5 sides).

Monocortical Miniplate Osteosynthesis

The theoretic basis of noncompression plating using miniplates and monocortical screws is explained. The technique is described and the range of clinical applications in maxillofacial trauma and elective maxillofacial osteotomies is indicated.

Champy plates in mandibular surgery

Champy plates are becoming more and more accepted for the fixation of simple mandibular fractures, fixation of middle face fractures and fixation after Le Fort osteotomies. This area of indication can be extended by their use in cases of complicated mandibular fractures, fixation of bone graft, and immediate reconstruction following removal of mandibular tumours. Experience shows that Champy plates have the following advantages over other methods: the plates are small, flexible, easily adaptable and the monocortical screws allow a rapid surgical application. Some examples are presented.

Practical orthodontics: A series of illustrated case-histories from practice. No. 1

Our aim was to assess the influence of internal fixation in skeletal stability on patients who had had counterclockwise rotation of the maxillomandibular complex and mandibular advancement procedures. We studied 60 records of 20 patients (14 female, 6 male), mean (range) age at operation 29 (16-50) years. The mean (range) postoperative follow-up was 15 (8-24) months. Sixty standard lateral cephalometric radiographs were randomly traced and digitised by one senior radiologist to estimate surgical and postoperative changes. Patients were divided into two groups, the first group (n = 10) of which had fixation with only 2.0 system plates (2 plates with monocortical screws alone) and the second (n = 10) of which had hybrid fixation (1 plate with monocortical screws and 2 or 3 bicortical bone screws). During operation the change in the mean occlusal plane with counterclockwise rotation was 9.4° (range -17.3 to -2.5 mm). The maxilla moved forward and upward. All the anterior mandibular measurements had advanced horizontally, the mean (range) being 17 (6.4 to 9.9) mm for the pogonion, and 17.6 (6.0 to 30.7) mm for the menton. At the longest follow-up period, there were significant long-term changes, but these were clinically acceptable (<2 mm). There was no significant difference between the two groups in postoperative stability or in the magnitude of the advancement and stability.