Biomechanical properties of volar hybrid and locked plate fixation in distal radius fractures

Design considerations for patient-specific bone fixation plates: a literature review

In orthopedic surgery, patient-specific bone plates are used for fixation when conventional bone plates do not fit the specific anatomy of a patient. However, plate failure can occur due to a lack of properly established design parameters that support optimal biomechanical properties of the plate.This review provides an overview of design parameters and biomechanical properties of patient-specific bone plates, which can assist in the design of the optimal plate.A literature search was conducted through PubMed and Embase, resulting in the inclusion of 78 studies, comprising clinical studies using patient-specific bone plates for fracture fixation or experimental studies that evaluated biomechanical properties or design parameters of bone plates. Biomechanical properties of the plates, including elastic stiffness, yield strength, tensile strength, and Poisson's ratio are influenced by various factors, such as material properties, geometry, interface distance, fixation mechanism, screw pattern, working length and manufacturing techniques.Although variations within studies challenge direct translation of experimental results into clinical practice, this review serves as a useful reference guide to determine which parameters must be carefully considered during the design and manufacturing process to achieve the desired biomechanical properties of a plate for fixation of a specific type of fracture.

Biomechanical properties of artificial bones made by Sawbones: A review

Biomedical engineers and physicists frequently use human or animal bone for orthopaedic biomechanics research because they are excellent approximations of living bone. But, there are drawbacks to biological bone, like degradation over time, ethical concerns, high financial costs, inter-specimen variability, storage requirements, supplier sourcing, transportation rules, etc. Consequently, since the late 1980s, the Sawbones® company has been one of the world's largest suppliers of artificial bones for biomechanical testing that counteract many disadvantages of biological bone. There have been many published reports using these bone analogs for research on joint replacement, bone fracture fixation, spine surgery, etc. But, there exists no prior review paper on these artificial bones that gives a comprehensive and in-depth look at the numerical data of interest to biomedical engineers and physicists. Thus, this paper critically reviews 25 years of English-language studies on the biomechanical properties of these artificial bones that (a) characterized unknown or unreported values, (b) validated them against biological bone, and/or (c) optimized different design parameters. This survey of data, advantages, disadvantages, and knowledge gaps will hopefully be useful to biomedical engineers and physicists in developing mechanical testing protocols and computational finite element models.

Retrospective Comparative Study of Clinical Outcomes and Cost-Effectiveness with Bone Substitutes on Volar Locking Plate Fixation of Unstable Distal Radial Fractures in the Elderly

Background: This multicenter retrospective study aimed to compare clinical outcomes and cost-effectiveness with bone substitutes on volar locking plate (VLP) fixation of unstable distal radial fractures (DRF) in the elderly. Methods: The data of 1,980 patients of ≥65 years of age who underwent surgery for the DRF with a VLP in 2015-2019 were extracted from a database (named TRON). Patients lost to follow-up or who received autologous bone grafting were excluded. The patients (n = 1,735) were divided into the VLP fixation alone (Group VLA) and VLP fixation with bone substitutes (Group VLS). Propensity score matching of background characteristics (ratio, 4:1) was performed. The modified Mayo wrist scores (MMWS) were evaluated as clinical outcomes. The implant failure rate, bone union rate, volar tilt (VT), radial inclination (RI), ulnar variance (UV) and distal dorsal cortical distance (DDD) were evaluated as radiologic parameters. We also compared the initial surgery cost and total cost for each group. Results: After matching, the backgrounds of Groups VLA (n = 388) and VLS (n = 97) were not significantly different. The MMWS values of the groups were not significantly different. Radiographic evaluation revealed no implant failure in either group. Bone union was confirmed in all patients in both groups. The VT, RI, UV and DDD values of the groups were not significantly different. The initial surgery cost and total cost in the VLS group were significantly higher than those in the VLA group ($3,515 vs. $3,068, p < 0.001). Conclusions: In patients of ≥65 years of age with DRF, the clinical and radiological outcomes of VLP fixation with bone substitutes did not differ from those of VLP fixation alone, yet the additional use of bone augmentation was associated with higher medical costs. The indications for bone substitutes should be more strictly considered in the elderly with DRF. Level of Evidence: Level IV (Therapeutic).

Are the Clinical Results of Locking Plate Fixation for Distal Radius Fractures Inferior in Patients over 80 Years of Age? A Multicentre (TRON Group) Study

Introduction

With the aging of the population, the proportion of distal radius fracture patients who are > 80 years of age is increasing. In this study, we compared the postoperative clinical and radiographic outcomes between super-elderly patients (age: ≥ 80 years) and middle-elderly (age: 65-79 years) who were treated with volar locking plate (VLP) fixation for distal radius fractures.

Patients and Methods

Patients of > 65 years of age with distal radius fractures treated by VLP fixation between 2015 and 2019, and who were followed for at least 6 months after surgery were included in our database (named TRON). Patients with open fractures, multiple-trauma, or who received fixation with implants other than a VLP were excluded. We evaluated postoperative complications, Mayo wrist score (MWS), and radiographic outcomes.

Results

We identified 589 patients in this study; 452 were 65-79 years of age (Group A) and 137 were ≥ 80 years of age (Group B). After propensity score matching, we evaluated 309 patients in Group A and 103 patients in Group B. The mean follow-up period was 10.7 ± 4.6 months. Twenty-eight patients (9.1%) in Group A and 5 patients in Group B (4.9%) experienced post-operative complications (non-significant: p = 0.212). The postoperative MWS at 1, 3, and 6 months, respectively, was 65.4 ± 11.7, 75.2 ± 11.0, and 79.6 ± 10.5 in Group A and 67.1 ± 9.61, 75.7 ± 10.7, and 80.6 ± 9.7 in Group B (non-significant: p = 0.418, 0.893, 0.452, respectively). The differences in volar tilt, radial inclination, ulnar variance between the postoperative and last follow-up radiographs did not differ between the two groups to a statistically significant extent (p = 0.053, 0.437, 0.529, respectively).

Conclusion

Our study showed that the clinical and radiographic outcomes of distal radius fractures treated with VLP in super-elderly patients were comparable to those in middle-elderly patients.

Distal radius fracture fixation using volar plate: A comparative study evaluating the biomechanical behavior of uni and bicortical distal screws

Extensor tendon ruptures caused by bicortical screws impingement following distal radius fracture fixation with volar plates are extensively reported in the literature. Thus, a biomechanical study comparing unicortical and bicortical fixations in intra-articular distal radius fracture models is critical in decision-making regarding distal radius fracture management. Forty-two synthetic radius models were fixed using a variable angle volar distal locking plate with seven screws. They were divided into 6 groups (n = 7): G1/G3/G4 unicortical fixation (75% of anteroposterior distal radius lenght); G2/G4/G6 bicortical fixation. Each group underwent a different mechanical test: axial compression (G1/G2), dorsal flexion (G3/G4), and volar flexion (G5/G6). The load application rate was 5 mm/min and 1000 cycles of 50 to 250 N at 1 Hz were performed between both static tests. Comparative results in the first static test, in the second static test, and in failure generally showed a very similar behavior. Models depicted similar behavior in the second static test when cyclic load was performed. Therefore, one can realize that stiffness differed during dorsal flexion only in the first static test. Maximum force to break the model in axial compression was greater in bicortical than in unicortical construct. Since biomechanical properties are similar, we recommend using unicortical distal locking screws in distal radius fracture fixation with volar plates to prevent extensor tendon ruptures.

Biomechanical evaluation of the stability of extra-articular distal radius fractures fixed with volar locking plates according to the length of the distal locking screw

Surgeons usually used short screws to avoid extensor tendon problems during volar locking plate fixation in distal radius fracture. However, the stability according to the length of distal locking screws have not been fully understood. We investigated this issue through finite element analysis and compression test using synthetic radius. Our results demonstrated that the bi-cortical full-length fixation does not contribute to the stiffness increase in the axial compression direction, and a reduction in length of up to more than 50% length can still provide similar stability to full-length screws. Our data can support that surgeon should undersize the distal screw.

The Effect of Plate Design on the Flexor Pollicis Longus Tendon After Volar Locked Plating of Distal Radial Fractures

BACKGROUND

Injury to and rupture of the flexor pollicis longus (FPL) tendon are known complications after volar locking plate fixation for distal radial fractures. Recent investigations have demonstrated that plate positioning contributes to the risk of tendon rupture; however, the impact of plate design has yet to be established. The purpose of this study was to compare FPL tendon-to-plate distance, FPL tendon-plate contact, and sonographic changes in the FPL tendon for 2 volar locking plate designs (ADAPTIVE compared with FPL) using ultrasound examination.

METHODS

We identified patients who underwent distal radial fracture fixation by 2 fellowship-trained hand surgeons with either standard (ADAPTIVE) or FPL plates. Patients were matched by age, sex, and Soong grade. Enrolled patients returned for a research-related office visit for a clinical examination and bilateral wrist ultrasound. We measured plate-tendon distance, plate-tendon contact, sonographic changes in the FPL tendon, and postoperative radiographic parameters in the operatively treated wrist and the uninjured wrist.

RESULTS

Forty patients with Soong grade-1 or 2 plate prominence underwent bilateral wrist ultrasound examination; all of the patients had distal radial fracture fixation, 20 with the standard volar locking plate and 20 with the FPL volar locking plate. Similar proportions of patients with the FPL plate (65%) and those with the standard plate (79%) had plate-tendon contact (p = 0.48); however, the FPL volar locking plate group had significantly less of the FPL tendon in contact with the volar plate than the standard volar locking plate group at wrist extension at both 0° (p < 0.001) and 45° (p < 0.001). There was no difference (p = 0.5) in the proportion of patients with sonographic changes in the FPL tendon between the FPL volar locking plate group (25%) and the standard volar locking plate group (21%). The postoperative volar tilt was significantly lower in patients with FPL plate-tendon contact (p = 0.01) and correlated moderately with the percentage of FPL tendon-plate contact at 0° (r = -0.51; p < 0.001) and 45° (r = -0.53; p < 0.001). There were no cases of tendon rupture in the cohort.

CONCLUSIONS

We found that the FPL volar locking plate and increased volar tilt significantly reduced the plate-tendon contact area compared with the standard volar locking plate. In our asymptomatic cohort, we were unable to find a difference in sonographic changes in the FPL tendon. Further studies are needed to determine the clinical importance of decreased tendon-plate contact area seen in modified volar locking plate designs.

LEVEL OF EVIDENCE

Therapeutic Level III. See Instructions for Authors for a complete description of Levels of Evidence.

Mechanical testing of scapular neck fracture fixation using a synthetic bone model

BACKGROUND

Trauma can fracture the scapular neck. Typically, a single plate along the lateral scapula border affixes the glenoid fragment to the scapula. This method is limited by difficulty in screw placement, frequent excessive soft tissue dissection, and risk for neurovascular injury. Substituting 2 smaller plates bridging the scapular neck mitigates these limitations, but no comparative mechanical data between techniques exists. Therefore, we compared the mechanical properties of two constructs securing a simulated scapular neck fracture.

METHODS

Twenty synthetic human scapulae underwent a templated scapular neck fracture. Repairs were performed with a single plate on the lateral scapular border (Column method), or two small plates parallel to the lateral border (Neck method). Measures of displacement, force, and stiffness were quantified during cyclic testing (20-150 N, 1 Hz, 1000 cycles) and loading to failure. Statistical comparisons were made with t-tests (p ≤ 0.050).

FINDINGS

The column constructs had higher displacements than neck constructs after 1000 cycles, but differences were small (mean) 0.18 (SD 0.01) vs. 0.15 (0.02) mm (p ≤ 0.004). Cyclic stiffness was 655 (43) and 790 (88) N/mm for the column and neck constructs, respectively (p ≤ 0.003). Both techniques performed comparably in failure loading: at 1 mm of gap reduction the compressive loads were 426 (61) N and 428 (48) N and stiffness was 354 (129) and 334 (80) N/mm for the column and neck constructs, respectively.

INTERPRETATION

Given the surgical advantages, the neck fixation may be more suitable without biomechanical compromise compared to traditional lateral column fixation.

Damage in a Distal Radius Fracture Model Treated With Locked Volar Plating After Simulated Postoperative Loading

PURPOSE

"Damage" is an engineering term defining a period between a state of material perfection and the onset of crack initiation. Clinically, it is a loss of fixation due to microstructural breakdown, indirectly measured as a reduction of stiffness of the bone-implant construct, normalized by the cross-sectional area and length of the bone. The purpose of this study was to characterize damage in a cadaver model of extra-articular distal radius fracture with dorsal comminution treated using 2-column volar distal radius plates.

METHODS

Ten matched distal radii were randomly divided into 2 groups: group I specimens were treated with a volar distal radius plate with an independent, 2-tiered scaffold design; group II specimens (contralateral limbs) were treated with a volar plate with a single-head design for enhanced ulnar buttressing. Specimens were cyclically loaded to simulate a 6-month postoperative load-bearing period. We report damage after a defined protocol of cyclical loading and load to failure simulating a fall on an outstretched hand.

RESULTS

Group II specimens experienced more damage under cyclic loading conditions than group I specimens. Group I specimens were stiffer than group II specimens under load-to-failure conditions. Ultimate force at failure in group I and group II specimens was not different. Specimens failed by plate bending (group I, n = 6/10; group II, n = 2/10) and fracture of the lunate facet (group I, n = 4/10; group II, n = 8/10).

CONCLUSIONS

Group I specimens had less screw cutout at the lunate facet than group II specimens under cyclic loading as indicated by lower damage measures and fewer facet fractures during load-to-failure testing. The overall strength of the construct is not affected by plate design.

CLINICAL RELEVANCE

Microstructural damage or a loss of fixation due to an overly rigid volar plate design may cause malunion or nonunion of fracture fragments and lead to bone-implant instability.

Fracture Gap Reduction With Variable-Pitch Headless Screws

PURPOSE

Fully threaded, variable-pitch, headless screws are used in many settings in surgery and have been extensively studied in this context, especially in regard to scaphoid fractures. However, it is not well understood how screw parameters such as diameter, length, and pitch variation, as well as technique parameters such as depth of drilling, affect gap closure.

METHODS

Acutrak 2 fully threaded variable-pitch headless screws of various diameters (Standard, Mini, and Micro) and lengths (16-28 mm) were inserted into polyurethane blocks of "normal" and "osteoporotic" bone model densities using a custom jig. Three drilling techniques (drill only through first block, 4 mm into second block, or completely through both blocks) were used. During screw insertion, fluoroscopic images were taken and later analyzed to measure gap reduction. The effect of backing the screw out after compression was evaluated.

RESULTS

Drilling at least 4 mm past the fracture site reduces distal fragment push-off compared with drilling only through the proximal fragment. There were no significant differences in gap closure in the normal versus the osteoporotic model. The Micro screw had a smaller gap closure than both the Standard and the Mini screws. After block contact and compression with 2 subsequent full forward turns, backing the screw out by only 1 full turn resulted in gapping between the blocks.

CONCLUSIONS

Intuitively, fully threaded headless variable-pitch screws can obtain compression between bone fragments only if the initial gap is less than the gap closed. Gap closure may be affected by drilling technique, screw size, and screw length. Fragment compression may be immediately lost if the screw is reversed.

CLINICAL RELEVANCE

We describe characteristics of variable-pitch headless screws that may assist the surgeon in screw choice and method of use.

A biomechanical comparison of the two- and four-hole side-plate dynamic hip screw in an osteoporotic composite femur model

OBJECTIVES

The objectives of this study were (1) to compare the axial and torsional stiffness of a dynamic hip screw with a two- and four-hole side-plate in a synthetic model of a healed and stable intertrochanteric femur fracture and (2) to evaluate the load to failure, as well as propensity to peri-implant fracture.

METHODS

Fourth-generation synthetic composite femur models, simulating osteoporotic bone, were implanted with 135° dynamic hip screws (DHS) with either a two- or four-hole side-plate with or without a stable intertrochanteric fracture. Specimens were cyclically loaded up to a nondestructive load to determine the axial and torsional stiffness. Constructs were then loaded to failure in axial compression emulating physiologic forces. Failure load and location of the peri-implant fractures were recorded.

RESULTS

Axial and torsional stiffness did not differ significantly between the two- and four-hole constructs in either model. Likewise, there was no significant difference in the load to failure. In the intact femurs, failure occurred either at the end of the plate at the distal screw or through the lag screw hole.

CONCLUSION

The results of this study demonstrate that DHS constructs with a two- or four-hole side-plate are biomechanically comparable with regard to axial and torsional stiffness and load to failure in an osteoporotic composite femur model. In a healed intertrochanteric fracture model, a two-hole construct did not appear to be more prone to peri-implant fracture. To date, a biomechanical comparison of these two implants with regard to torsional forces has not been reported.

Biomechanical Comparison of Intrapelvic and Extrapelvic Fixation for Acetabular Fractures Involving the Quadrilateral Plate

OBJECTIVES

Elderly patients represent the fastest growing and most difficult to treat population sustaining acetabular fractures. When treated surgically, isolated extrapelvic or combined intrapelvic-extrapelvic constructs may be used. No biomechanical or clinical study has compared the merits of these 2 techniques in cadaveric models. This research aims to biomechanically quantify the additional benefit of intrapelvic fixation to a standard extrapelvic fixation construct.

METHODS

Ten cadaveric pelves underwent standardized anterior column and quadrilateral plate fracture creation. One hemipelvis from each subject received isolated extrapelvic fixation, whereas the other received adjunctive intrapelvic fixation. Specimens were then subjected to a 50% of body weight (BW) nondestructive stiffness test followed by loading to failure. For the 50% BW test, displacement at 50% BW and stiffness were calculated. For the load to failure test, stiffness, elastic energy, and plastic energy were calculated. Yield point, force at clinical failure (defined at 2 mm of displacement), and maximum force were also identified. A Wilcoxon matched-pairs t test was used to compare fixation groups.

RESULTS

The addition of an intrapelvic plate improved construct performance for all test parameters. A statistically significant difference (P < 0.05) was reached for yield force, maximum force, and plastic energy.

CONCLUSIONS

These findings demonstrate that the addition of intrapelvic plating may offer distinct advantages in prevention of catastrophic construct failure in situations in which significant lateral to medial force is applied to the greater trochanter such as patient falling.

Impact of double-tiered subchondral support procedure with a polyaxial locking plate on the stability of distal radius fractures using fresh cadaveric forearms: Biomechanical and radiographic analyses

BACKGROUND

The present study compared the changes in biomechanical and radiographic properties under cyclic axial loadings between the 'double-tiered subchondral support' (DSS) group (wherein two rows of screws were used) and the 'non-DSS' (NDSS) group (wherein only one row of distal screws was used) using cadaveric forearm models of radius fractures fixed with a polyaxial locking plate.

MATERIAL AND METHODS

Fifteen fresh cadaveric forearms were surgically operated to generate an Arbeitsgemeinschaft für Osteosynthesefragen (AO) type 23-C2 fracture model with the fixation of polyaxial volar locking plates. The model specimens were randomized into two groups: DSS (n = 7) and NDSS (n = 8). Both the groups received 4 locking screws in the most distal row, as is usually applied, whereas the DSS group received 2 additional screws in the second row inserted at an inclination of about 15° to support the dorsal aspect of the dorsal subchondral bone. Cyclic axial compression test was performed (3000 cycles; 0-250 N; 60 mm/min) to measure absolute rigidity and displacement, after 1, 1000, 2000 and 3000 cycles, and values were normalized relative to cycle 1. These absolute and normalized values were compared between those two groups. Radiographic images were taken before and after the cyclic loading to measure changes in volar tilt (ΔVT) and radial inclination (ΔRI).

RESULTS

The DSS group maintained significantly higher rigidity and lower displacement values than the NDSS group during the entire loading period. Radiographic analysis indicated that the ΔVT values of the DSS group were lower than those of the NDSS group. In contrast, the fixation design did not influence the impact of loading on the ΔRI values.

CONCLUSIONS

Biomechanical and radiographic analyses demonstrated that two rows of distal locking screws in the DSS procedure conferred higher stability than one row of distal locking screws.

Optimal Positioning for Volar Plate Fixation of a Distal Radius Fracture: Determining the Distal Dorsal Cortical Distance

Distal radius fractures are currently among the most common fractures of the musculoskeletal system. With a population that is living longer, being more active, and the increasing incidence of osteoporosis, these injuries will continue to become increasingly prevalent. When operative fixation is indicated, the volar locking plate has recently become the treatment of choice. However, despite its success, suboptimal position of the volar locking plate can still result in radiographic loss of reduction. The distal dorsal cortical distance is being introduced as an intraoperative radiographic tool to help optimize plate position and minimize late loss of fracture reduction.

The influence of distal screw length on the primary stability of volar plate osteosynthesis--a biomechanical study

Background

Extensor tendon irritation is one of the most common complications following volar locking plate osteosynthesis (VLPO) for distal radius fractures. It is most likely caused by distal screws protruding the dorsal cortex. Shorter distal screws could avoid this, yet the influence of distal screw length on the primary stability in VLPO is unknown. The aim of this study was to compare 75 to 100 % distal screw lengths in VLPO.

Methods

A biomechanical study was conducted on 11 paired fresh-frozen radii. HRpQCT scans were performed to assess bone mineral density (BMD) and bone mineral content (BMC). The specimens were randomized pair-wise into two groups: 100 % (group A) and 75 % (group B) unicortical distal screw lengths. A validated fracture model for extra-articular distal radius fractures (AO-23 A3) was used. Polyaxial volar locking plates were mounted, and distal screws was inserted using a drill guide block. For group A, the distal screw tips were intended to be flush or just short of the dorsal cortex. In group B, a target screw length of 75 % was calculated. The specimens were tested to failure using a displacement-controlled axial compression test. Primary biomechanical stability was assessed by stiffness, elastic limit, and maximum force as well as with residual tilt, which quantified plastic deformation.

Results

Nine specimens were tested successfully. BMD and BMC did not differ between the two groups. The mean distal screw length of group A was 21.7 ± 2.6 mm (range: 16 to 26 mm), for group B 16.9 ± 1.9 mm (range: 12 to 20 mm). Distal screws in group B were on average 5.6 ± 0.9 mm (range: 3 to 7 mm) shorter than measured. No significant differences were found for stiffness (706 ± 103 N/mm vs. 660 ± 124 N/mm), elastic limit (177 ± 25 N vs. 167 ± 36 N), maximum force (493 ± 139 N vs. 471 ± 149 N), or residual tilt (7.3° ± 0.7° vs. 7.1° ± 1.3°).

Conclusion

The 75 % distal screw length in VLPO provides similar primary stability to 100 % unicortical screw length. This study, for the first time, provides the biomechanical basis to choose distal screws significantly shorter then measured.

Electronic supplementary material

The online version of this article (doi:10.1186/s13018-015-0283-8) contains supplementary material, which is available to authorized users.

Composite bone models in orthopaedic surgery research and education

Composite bone models are increasingly used in orthopaedic biomechanics research and surgical education-applications that traditionally relied on cadavers. Cadaver bones are suboptimal for many reasons, including issues of cost, availability, preservation, and inconsistency between specimens. Further, cadaver samples disproportionately represent the elderly, whose bone quality may not be representative of the greater orthopaedic population. The current fourth-generation composite bone models provide an accurate reproduction of the biomechanical properties of human bone when placed under bending, axial, and torsional loads. The combination of glass fiber and epoxy resin components into a single phase has enabled manufacturing by injection molding. The high level of anatomic fidelity of the cadaver-based molds and negligible shrinkage properties of the epoxy resin results in a process that allows for excellent definition of anatomic detail in the cortical wall and optimized consistency of features between models. Recent biomechanical studies of composites have validated their use as a suitable substitute for cadaver specimens.

Biomechanical analysis of a volar variable-angle locking plate: the effect of capturing a distal radial styloid fragment

PURPOSE

Variable-angle volar locked constructs for distal radius fractures are a recent treatment addition. This study sought to biomechanically evaluate a variable-angle volar locking plate as compared with a fixed-angle construct.

METHODS

We created 2 different AO-C3 osteotomies in fourth-generation synthetic composite distal radiuses and labeled them proximal and distal. The distal osteotomy consisted of a smaller radial styloid fragment. We then fixed both sets of specimens with either a fixed-angle or variable-angle volar locking construct. We tested samples in axial compression with regard to cyclical loading and load to failure. Articular stepoff, stiffness, and load to failure data were then analyzed.

RESULTS

Neither the proximal nor the distal osteotomy groups showed articular failure after cyclic loading, significant loss of stiffness over cycling, or superior stiffness compared with the other. After load to failure in the proximal osteotomy, 1 of 8 fixed-angle and none of 8 variable-angle constructs had articular failure, whereas in the distal osteotomy, all 8 fixed-angle and none of 8 variable-angle constructs had articular failure.

CONCLUSIONS

Variable-angle and fixed-angle volar locked fixation of unstable intra-articular distal radius fractures in fourth-generation composite radii provide mechanically sound constructs with high load to failure values and no loss of stiffness over testing. The variable-angle construct exhibited excellent resistance to articular stepoff at load to failure and no loss of stiffness throughout cyclic loading, and did not exhibit significantly less overall stiffness compared with fixed-angle constructs. The variable-angle fixation exhibited a distinct mechanical advantage over fixed-angle fixation in the setting of a smaller radial styloid fragment.

CLINICAL RELEVANCE

Variable-angle constructs could be expected to hold up to standard loads in the postoperative period as well as traditional fixed-angle devices. The additional cost associated with variable-angle constructs may be warranted when treating distal radius fractures with radial styloid fragments, owing to the fragment-specific fixation allowed by customized screw placement.

The effects of screw length on stability of simulated osteoporotic distal radius fractures fixed with volar locking plates

PURPOSE

Volar plating for distal radius fractures has caused extensor tendon ruptures resulting from dorsal screw prominence. This study was designed to determine the biomechanical impact of placing unicortical distal locking screws and pegs in an extra-articular fracture model.

METHODS

We applied volar-locking distal radius plates to 30 osteoporotic distal radius models. We divided radiuses into 5 groups based on distal locking fixation: bicortical locked screws, 3 lengths of unicortical locked screws (abutting the dorsal cortex [full length], 75% length, and 50% length to dorsal cortex), and unicortical locked pegs. Distal radius osteotomy simulated a dorsally comminuted, extra-articular fracture. We determined each construct's stiffness under physiologic loads (axial compression, dorsal bending, and volar bending) before and after 1,000 cycles of axial conditioning and before axial loading to failure (2 mm of displacement) and subsequent catastrophic failure.

RESULTS

Cyclic conditioning did not alter the constructs' stiffness. Stiffness to volar bending and dorsal bending forces were similar between groups. Final stiffness under axial load was statistically equivalent for all groups: bicortical screws (230 N/mm), full-length unicortical screws (227 N/mm), 75% length unicortical screws (226 N/mm), 50% length unicortical screws (187 N/mm), and unicortical pegs (226 N/mm). Force at 2-mm displacement was significantly less for 50% length unicortical screws (311 N) compared with bicortical screws (460 N), full-length unicortical screws (464 N), 75% length unicortical screws (400 N), and unicortical pegs (356 N). Force to catastrophic fracture was statistically equivalent between groups, but mean values for pegs (749 N) and 50% length unicortical (702 N) screws were 16% to 21% less than means for bicortical (892 N), full-length unicortical (860 N), and 75% length (894 N) unicortical constructs.

CONCLUSIONS

Locked unicortical distal screws of at least 75% length produce construct stiffness similar to bicortical fixation. Unicortical distal fixation for extra-articular distal radius fractures should be entertained to avoid extensor tendon injury because this technique does not appear to compromise initial fixation.

CLINICAL RELEVANCE

Using unicortical fixation during volar distal radius plating may protect extensor tendons without compromising fixation.