Percutaneous insertion of a proximal humeral locking plate: an anatomic study

This cadaveric study sought to evaluate the feasibility of applying a locking proximal humerus plate with a novel minimally invasive technique. A unique pre-contoured locking plate was placed on cadaveric proximal humeri through a described minimally invasive approach. Proximity of the plate and screws to the axillary nerve and their respective surgical tracks were quantified. Safe screw hole placement with respect to the axillary nerve was determined. Risk of entrapment of the nerve beneath the plate was evaluated. Three of the holes near the middle of the locking plate consistently intersected the course of the axillary nerve and were unsafe for percutaneous placement of the screws. The axillary nerve could be palpated during the course of surgery and easily protected from injury. No entrapment of the axillary nerve occurred using this minimally invasive technique. The screw-in locking guide cannot be used with this technique as it caused tenting of the axillary nerve. Placement of a locking proximal humerus plate via a minimally invasive lateral trans-deltoid approach is safe if the locking screws are limited to superior and inferior holes. This can be done safely without entrapment of the axillary beneath the plate. Plate fixation of proximal humerus fractures may now be more desirable with the use of this approach.     

Anatomic study of the axillary nerve in a Chinese cadaveric population: correlation of the course of the nerve with proximal humeral fixation with intramedullary nail or external skeletal fixation

Introduction  

The axillary nerve can be injured during external skeletal fixation with Schanz screws or proximal locking screws of intramedullary nails. Being aware of the axillary nerve’s anatomic relationship to the proximal humerus is vital for avoiding complications.     

A systematic review of locking plate fixation of proximal humerus fractures

Purpose

Technique for the fixation of two, three, and four part proximal humerus fractures has rapidly shifted towards the use of specially contoured proximal humerus locking plates. The purpose of this study is to evaluate the short to medium term functional results and common complications associated with the fixation of proximal humerus fractures with locking plates.

Methods

The PubMed and EMBASE databases were used to perform a systematic review of the English literature to assess the functional results and complications associated with proximal humerus locking plates. Our inclusion criteria were proximal humerus fracture due to trauma (excluding pathologic fractures), patients greater than 18 years of age, more than 15 patients in the study or subgroup of interest, at least 18 months follow-up, at least one relevant functional outcome score, and quality outcome score of at least 5/10. Studies that did not meet these criteria were excluded. All institutional, author, and journal information was concealed to minimize reviewer bias.

Results

Twelve studies including 514 patients met the inclusion criteria. At most recent follow-up patients achieved a mean Constant score of 74 and a mean DASH score of 27. The overall rate of complications was 49% including varus malunion, 33% excluding varus malunion, and reoperation rate was 14%. The most common complications included varus malunion 16%, AVN 10%, screw perforation of the humeral head into the joint 8%, subacromial impingement 6%, and infection 4%.

Discussion

Fixation of proximal humerus fractures with proximal humerus locking plates is associated with a high rate of complications and reoperation. Further study is needed to determine what technical errors and patient characteristics are risk factors for failure of this now common fixation technique.     

Biomechanical in vitro assessment of screw augmentation in locked plating of proximal humerus fractures

Introduction

Proximal humerus fracture fixation can be difficult because of osteoporosis making it difficult to achieve stable implant anchorage in the weak bone stock even when using locking plates. This may cause implant failure requiring revision surgery. Cement augmentation has, in principle, been shown to improve stability. The aim of this study was to investigate whether augmentation of particular screws of a locking plate aimed at a region of low bone quality is effective in improving stability in a proximal humerus fracture model.

Materials and methods

Twelve paired human humerus specimens were included. Quantitative computed tomography was performed to determine bone mineral density (BMD). Local bone quality in the direction of the six proximal screws of a standard locking plate (PHILOS, Synthes) was assessed using mechanical means (DensiProbe™). A three-part fracture model with a metaphyseal defect was simulated and fixed with the plate. Within each pair of humeri the two screws aimed at the region of the lowest bone quality according to the DensiProbe™ were augmented in a randomised manner. For augmentation, 0.5 ml of bone cement was injected in a screw with multiple outlets at its tip under fluoroscopic control. A cyclic varus-bending test with increasing upper load magnitude was performed until failure of the screw–bone fixation.

Results

The augmented group withstood significantly more load cycles. The correlation of BMD with load cycles until failure and BMD with paired difference in load cycles to failure showed that augmentation could compensate for a low BMD.

Discussion and conclusion

The results demonstrate that augmentation of screws in locked plating in a proximal humerus fracture model is effective in improving primary stability in a cyclic varus-bending test. The augmentation of two particular screws aimed at a region of low bone quality within the humeral head was almost as effective as four screws with twice the amount of bone cement. Screw augmentation combined with a knowledge of the local bone quality could be more effective in enhancing the primary stability of a proximal humerus locking plate because the effect of augmentation can be exploited more effectively limiting it to the degree required.     

Versorgung von Humeruskopffrakturen

Objectives

Placement of a proximal humerus locking plate through a percutaneous transdeltoid approach bears the advantages of a minimally invasive approach but may compromise the anterior branches of the axillary nerve. This anatomic study aimed to develop a risk profile for 6 types of modern proximal humerus locking plates as to their interference with the axillary nerve.

Materials and methods

In this study six different implants (Arthrex®, DePuy®, Königsee®, Smith & Nephew®, Stryker® and Synthes®) were placed on the intact proximal humerus of 33 embalmed cadaveric upper extremities and the relative positioning between the axillary nerve and the screw holes was determined.

Results

All locking plates displayed an area of risk which concerned 3 out of 7 (Arthrex®), 4 out of 10 (DePuy®), 2 out of 9 (Königsee®), 3 out of 11 (Smith & Nephew®), 3 out of 11 (Stryker®) and 6 out of 12 (Synthes®) screw holes of the plate.

Conclusions

Using the anterolateral percutaneous deltoid splitting approach the relative position of the axillary nerve to the holes of a specific implant is of relevance for avoidance of iatrogenic lesions to the nerve.     

Use of a “hybrid” locking plate for complex metaphyseal fractures and nonunions about the humerus

Purpose

To review one surgeon's experience with a novel type of “hybrid” locking plate (which has both 3.5 mm and 4.5 mm locking holes) for difficult fractures of the meta-diaphyseal humeral shaft.

Methods

Over a 2-year period, 24 patients who presented with a metaphyseal humeral fracture or nonunion (proximal or distal) were treated surgically by a single surgeon. A “hybrid” locking plate containing 3.5 mm locking holes on one end and 4.5 mm locking holes on the other end (Metaphyseal plate, Synthes, Paoli, Pa) was used in all patients. The selection of this implant was based on fracture location and bone quality. Fractures were operated on through an anterolateral or direct posterior approach. All fractures were secured with a minimum of three 4.5 mm screws on one side of the fracture and three 3.5 mm screws on the other side. All patients were treated with a similar post-operative protocol for early range of shoulder and elbow motion.

Results

Three patients were lost to follow-up. The cohort consisted of 15 women and 6 men with a mean age of 49 years (range 18-78). There were 14 acute fractures and 7 nonunions. Twelve fractures involved the distal metaphyseal segment and 9 involved the proximal metaphyseal segment. Twenty-two patients completed a minimum 6-month clinical and radiographic follow-up and form the basis for this report. All 21 patients healed their fractures or nonunions at a mean of 4.5 months. There were no infections or hardware failures. In every case the “hybrid” nature of the plate design was felt to be advantageous.

Conclusion

This “second generation” metaphyseal locking plate, which affords the surgeon the ability to place a greater number of smaller calibre screws within a short bone segment, while using traditional large fragment screw fixation in the longer segment, is clearly an improvement in plate design. Meta-diaphyseal upper extremity long bones may serve as the most ideal location for this implant.     

Different stabilisation techniques for typical acetabular fractures in the elderly—A biomechanical assessment

Objectives

The tremendous increase of acetabular fractures in the elderly provides new challenges for their surgical treatment. The aim of this study was to evaluate the biomechanical properties of conventional and newly developed implants for the stabilisation of an anterior column combined with posterior hemitransverse fracture (ACPHTF), which represents the typical acetabular fracture in the elderly.

Methods

Using a single-leg stance model we analysed four different implant systems for the stabilisation of ACPHTFs in synthetic and cadaveric pelvises. Applying an increasing axial load, fracture dislocation was analysed with a new multidirectional ultrasonic measuring system. Results of the different implant systems were compared by Scheffé post hoc test and one-way ANOVA.

Results

In synthetic pelvises, the standard reconstruction plate fixed by 3 periarticular long screws and a new titanium fixator with multidirectional interlocking screws were associated with significantly less dislocation of the fractured quadrilateral plate of the acetabulum when compared to a standard reconstruction plate fixed by only one periarticular long screw and a locking reconstruction plate. No significant differences between the different osteosynthesis techniques could be observed in cadaver pelvises, probably due to a heterogeneous bone quality.

Conclusions

We conclude that the plate fixation by positioning of periarticular long screws as well as the multidirectional positioning of interlocking screws account for the most sufficient fracture stabilisation of ACPHTFs under experimental conditions.     

The percutaneous antegrade scaphoid fracture fixation—A safe method?

Introduction

The object of this study was to assess the risk of injury to tendons, nerves and vessels in percutaneous antegrade scaphoid fracture fixation.

Methods

Forty cadaveric forearms were used in this study. A guide wire for cannulated headless compression screws was inserted percutaneously in each scaphoid according to established surgical technique.

Results

No nerve or vessel injuries were observed. Tendons however were injured in 5 out of the 40 specimens. This included the extensor pollicis longus tendon in two specimens, the extensor carpi radialis tendon in two specimens and the extensor digitorum tendon in one specimen.

Conclusions

Soft tissue injuries may be avoided by extending the skin incision and performing blunt dissection down to guide wire and screw entry point. In this manner, dorsal antegrade fixation of scaphoid fractures by using cannulated headless compression screws can be considered to be a safe and reliable technique for fixation of scaphoid fractures.     

Complications and Outcomes of Minimally Invasive Percutaneous Plating for Proximal Humeral Fractures

Background

The minimally invasive plate osteosynthesis (MIPO) technique using periarticular locking plates may be a good option for the repair of displaced proximal humeral fractures. However, axillary nerve complications related to this technique may be underestimated. The purpose of this study is to evaluate the outcomes of the minimally invasive plating, focusing on the complications.

Methods

The records of 21 consecutive patients treated for proximal humerus fractures using the MIPO technique with locking plates were retrospectively reviewed. These patients were treated between March 2009 and March 2011 with a minimum one-year follow-up. The clinical function, complications, and radiological bony union were evaluated.

Results

All of the patients, with one exception, showed at least 90 degrees of flexion and abduction at the shoulder joint six months postoperatively. The average Constant scores at three months, six months, and one year follow-ups were 74.0 (range, 62 to 90), 79.4 (range, 64 to 91), and 82.7 (range, 66 to 92), respectively. All of the patients achieved bony union within the average of 3.2 months (range, 2 to 6 months). There was one case of delayed union, one case of intra-articular screw penetration, and one case of axillary nerve paresis (incomplete injury), which did not completely recover during the one year of follow-up.

Conclusions

The MIPO technique using periarticular locking plates is a useful option for the treatment of selected cases of displaced proximal humeral fractures. However, nerve complications such as axillary nerve paresis should be considered along with implant-related complications when choosing patients for minimally invasive plating.     

Assessment of axillary nerve function and functional outcome after fixation of complex proximal humeral fractures using the extended deltoid-splitting approach

Background and purpose

The extended deltoid-splitting approach was developed as an alternative to the deltopectoral approach in the treatment of three- and four-part proximal humeral fractures. The aim of our prospective study was to determine whether this approach was associated with evidence of nerve injury, functional deficits or other complications in these cases, during the first year following reconstruction.

Methods

Over a 1-year-period, we treated 14 people (median age 59 years) with open reduction and plate fixation using the extended deltoid-splitting approach. All were prospectively reviewed clinically and radiologically during the first year after surgery. Functional testing involved three scoring systems, spring balance testing of deltoid power, dynamic muscle function testing and, at 1 year, electrophysiological assessment of axillary nerve function.

Results

Of the 14 fractures, 13 united without complications and with comparatively minor residual functional deficits. Of these 13 cases, 1 showed slight neurogenic change in the anterior deltoid but no evidence of anterior deltoid paralysis. In the remaining case, osteonecrosis of the humeral head developed 9 months after surgery and functional scores were poor, but without evidence of nerve injury on electrophysiological testing.

Conclusions

This technique is a useful alternative in the treatment of complex proximal humeral fractures, providing good access for reduction and implant placement without adverse effects.     

Injury to the axillary artery, a complication of fixation using a locking plate

The proximal humerus internal locking system is an internal fixation device consisting of a low-profile plate and locking screws, which is used for the fixation of three- and four-part fractures of the proximal humerus. We describe a case in which the screws cut out of the humeral head causing injury to the axillary artery necessitating urgent removal of the implant.     

Surgical treatment of two-part fractures of the proximal humerus: Comparison of fixed-angle plate osteosynthesis and Zifko nails

Introduction

Closed reduction and percutaneous pinning using Zifko nails offer the advantage of a minimal soft-tissue dissection but have been criticised for limited stability and secondary fracture dislocation. Angular stable plate osteosynthesis enables anatomic reduction, but carries the risk of soft-tissue traumatisation and consecutive humeral head necrosis. The present study compares the clinical and radiological outcome of patients with dislocated two-part fractures of the proximal humerus, who were treated with either Zifko nails or angular stable plate fixation.

Material and methods

A matched-pair analysis was performed and patient groups were matched according to age (±3 years), sex and fracture type. As many as 11 pairs of patients with a minimum follow-up of 3 years were formed and investigated radiographically and clinically using the Constant score (CS) and the visual analogue scale (VAS) for the patients’ satisfaction.

Results

At the time of follow-up, the absolute CS was 83 points in the PHP group and 78 points in the Zifko group (n.s.). Neither in the age and gender-corrected CS was found a significant difference between the study groups (PHP 104 ± 29, Zifko 95 ± 17, n.s.) nor in terms of subjective patient satisfaction (PHP 6.54; Zifko 7.8, n.s.). The complication rate was also comparable in both groups.

Conclusion

In conclusion, Zifko nailing represents a cost-effective minimally invasive surgical method with a complication rate and clinical outcome comparable to that after angular stable osteosynthesis by angular stable plate fixation in the treatment of two-part fractures of the proximal humerus.     

Predicting failure after surgical fixation of proximal humerus fractures

Background

Several studies reported high failures rates after internal fixation of proximal humerus fractures. Loss of reduction and screw cut-out are the most common reasons for revision surgery. Several risk factors for failure have been described in the literature. The aim of the present study was to assess risk factors for failure after surgical fixation of unstable proximal humerus fractures in a multivariate setup.

Methods

Two different surgical techniques (PHILOS locking plate and Humerusblock) were used. In the PHILOS group, every kind of postoperative relative movement between the implant and the humeral head or shaft was defined as failure. In the Humerusblock group, postoperative movement between the humeral head and the shaft in terms of angulation or translational displacement was defined as failure. The following parameters were assessed: age, gender, cancellous bone mineral density (BMD) of the humeral head, fracture type, medial metaphyseal comminution, medial metaphyseal head extension, initial angulation of the humeral head in the frontal plane, initial anteversion of the humeral head, medial hinge displacement, maximum displacement of the tuberosities with respect to the head, surgical technique, anatomic reconstruction and restoration of the medial cortical support.

Results

The following parameters were found to have a significant influence on the failure rate: age, local BMD, anatomic reduction, and restoration of the medial cortical support. The failure rate significantly increased with the number of risk factors.

Conclusion

Preoperative assessment of the local BMD and the patients’ biological age as well as intraoperative anatomic reduction and restoration of the medial cortical support are the essentials for successful surgical fixation of proximal humerus fractures. Multifragmentary fracture patterns in old patients with low local BMD are prone for fixation failure. If the surgeon is not able to achieve anatomic reduction and restoration of the medial cortical support intraoperatively in this situation, adjustments such as augmentation or primary arthroplasty should be considered.     

Risk of injury to the axillary nerve during antegrade proximal humeral blade nail fixation – An anatomical study

Introduction

The antegrade intramedullary Locking Blade Nail (Marquardt, Germany) is a device aimed at improving purchase in the humeral head and reducing varus displacement by providing medial buttress support and triangular stability within the humeral head. The aim of this study is to measure the relationship of the proximal fixation screws to the axillary nerve.

Methods

13 whole cadavers underwent insertion of an antegrade proximal humeral blade nail via a deltoid split approach to both shoulders. The anatomic proximity of the anterior branch of the axillary nerve to the screws was measured following soft tissue dissection and inspection of the nerve.

Results

The mean distance of the nerve from the anterolateral acromion was 62 mm (range 45–81 mm). The nerve lay closest to the distal blade fixation screw 4.9 mm (range 0–19 mm). In three cases the nerve lay directly underneath the washer and in all three cases there was macroscopic evidence of damage to the nerve. In 5 cases the nerve travelled obliquely in a cranial direction to lie 1.8 mm (range 0–3 mm) from the distal blade fixation screw, in 2 of these cases the nerve lay beneath the washer.

Conclusion

The anterior branch of the axillary nerve is placed at risk during insertion of the locking screws despite use of protection sleeves and trocars. We advocate that when using antegrade intramedullary nails that incorporate an inferomedial calcar screw an extended anterolateral acromial approach is undertaken.     

Sub-muscular plating of the humerus: An emerging technique

Objective

The purpose of the present study was to evaluate percutaneous sub-muscular internal fixation using a locked screw methodology for treatment of diaphyseal humeral fractures.

Methods

Inclusion criteria were multiple extremity fractures, open fractures, neurovascular injuries, additional ipsilateral upper extremity fractures, the inability to obtain a satisfactory closed reduction and isolated fractures with circumstances that prevented effective bracing. Exclusion criteria were immaturity, neoplasm, infection and intra-articular extensions in the same bone. Outcome measures included clinical and radiographic healing, complications, elbow and shoulder symptoms, range of motion (ROM) and Constant-Murley (CM) scores.

Results

Thirty-one patients with 32 fractures were evaluated with a mean follow-up of 16 months (3-38 months). There was radiographic healing in 31 out of the 32 fractures; the non-union was revised to open plating at 6 months and healed uneventfully. Hardware complications included two construct disengagements; one patient was revised and healed, and the other achieved union with bracing. Neurovascular complications included one preoperative nerve palsy that recovered by 3 months, two partial to complete postoperative nerve palsies that recovered by 6 months, and one intact-to-complete nerve palsy due to a bone fragment that required decompression with full recovery by 3 weeks. All patients had functional ROM with a mean CM score of 88. There were no elbow complaints and minor shoulder dysfunction occurred in two patients with ipsilateral shoulder injuries. The rate of neurovascular complications was comparable to open plating techniques and all patients had full recovery.

Conclusion

We feel sub-muscular anterior plating of the humerus using locking screw technology is a viable and useful method for diaphyseal humeral fractures.     

The treatment of established non-union of the proximal humerus using the Polarus locking intramedullary nail

Introduction:

Non-union following fracture of the proximal humerus is not uncommon, particularly in the elderly. This can be associated with significant morbidity due to pain, instability and functional impairment. The Polarus device (Acumed) is a locked, antegrade intramedullary nail designed to stabilize displaced 2-, 3- and 4-part fractures of the proximal humerus. We report our experience with the Polarus nail for the treatment of established non-union of the proximal humerus.

Materials and Methods:

A total of 7 Polarus nails were inserted for the treatment of non-union of the proximal humerus between June 2000 and July 2007. Each fracture site was opened, debrided, stabilized with a Polarus nail and then grafted with autologous cancellous iliac crest bone. The time between injury and surgery ranged from 6 to 102 months. One patient had undergone previous fixation of her fracture using Rush intramedullary rods. All patients were females, and mean age at surgery was 63.6 years (range, 49-78 years). A retrospective review of notes and radiographs was carried out. Patients were reviewed at varying intervals postoperatively (range, 13-68 months) and assessed using the Constant shoulder-scoring system.

Results:

All un-united fractures progressed to union. There were no wound complications and no postoperative nerve palsies. Functional outcome was good, even in those cases with a long interval between injury and surgery. The mean Constant score was 63 (range, 54-81). Migration of a single proximal locking screw was seen in 2 patients, and these screws required removal at 5 and 12 months, respectively, postoperatively.

Conclusion:

In our experience, a locked proximal humeral nail used in conjunction with autologous bone grafting is an excellent device for the treatment of proximal humerus non-unions.     

The relationship of proximal locking screws to the axillary nerve during antegrade humeral nail insertion of four commercially available implants

OBJECTIVE: The purpose of this study was to determine the proximity of proximal interlocking mechanisms in 4 current antegrade humeral nails to the axillary nerve and its branches. DESIGN: Cadaveric study. SETTING: Anatomy laboratory. MAIN OUTCOME MEASURE: Anatomic relationships. METHODS: Four humeral nail designs (labeled SS, SL, SZ, and SN) were each inserted in successive antegrade fashion in 10 cadaveric upper extremity specimens. Three variables were measured: from acromion to the axillary nerve, from acromion to entry sites of proximal locking devices, and from locking devices to axillary nerves and their branches. RESULTS: In nail SS, the proximally directed oblique locking screw came into contact with the ascending branch of the axillary nerve in 6 of 10 specimens. Mean distance from spiral blades in nails SS and SL were 26 mm to the axillary nerve and 16 mm to its ascending branch. Interlocking screws for nails SZ, SN, and SL did not violate the axillary nerve or its branches in any specimen. Mean distance from lateral acromion to the axillary nerve measured 58.7 mm. CONCLUSION: Nail SS's oblique locking screw may injure the ascending branch of the axillary nerve. Three of the 4 nails tested did not endanger the axillary nerve. However, when transverse proximal locking screws are inserted from a lateral-to-medial direction, they may endanger an arborized axillary nerve. Blunt dissection should be performed with a visible path to bone before instrumentation to reduce the risk of axillary nerve injury.     

Anterior minimally invasive bridge-plate technique for treatment of humeral shaft nonunion

Background

The present study introduces a new surgical technique and the results of a case series of patients with humeral shaft nonunion.

Materials and methods

Fifteen patients with diagnosis of diaphyseal nonunion of humerus were operated by a bridge-plate technique. A 4.5-mm plate is slid on the anterior surface of the humerus, submuscular to the brachial muscle. With the plate over the anterior surface of the humerus, screws are inserted from anterior to posterior on the ends of the plate. When there is a small bone gap, an iliac autologous graft is inserted. Minimum follow-up was 1 year.

Results

Bone healing was obtained in all patients: 1.5 months postoperatively in 11 patients, 2 months in 3 patients, and 3 months in 1 patient. There were no postoperative infections, there was one case with loosening of the screws and plate, and there were no nerve injuries.

Conclusions

The present technique avoids wide dissection, radial nerve isolation, and periosteum stripping. The anterior minimally invasive bridge-plate technique for treatment of humeral shaft nonunion is a safe procedure and obtained bone healing in all patients in this series.     

Shoulder reanimation in posttraumatic brachial plexus paralysis

Introduction

Posttraumatic brachial plexus paralysis invariably involves the upper roots leading to paralysis of the shoulder region musculature. Early neurotisation of the suprascapular and the axillary nerve should be one of the priorities in plexus reconstruction in order to reanimate the shoulder.

Patients and methods

From 1998 to 2007, 78 patients with posttraumatic brachial plexus palsy were operated in our department. Forty-three patients presented with supraclavicular lesions with involvement of C5 and C6 roots in all cases. Reconstruction of the shoulder function was achieved with neurotisation of the suprascapular nerve in 41 patients. Extraplexus donors were utilised in 34 patients, while intraplexus donors via nerve grafts in 7 patients. Neurotisation of the axillary nerve was performed in 25 patients, utilising intraplexus donors in 16 patients, extraplexus donors in 4, and combination of intraplexus and extraplexus donors in 5 patients.

Results

Suprascapular nerve neurotisation gave good or excellent results (supraspinatus > M3+ or shoulder abduction > 40°) in 35 patients. Intraplexus donors regained good or excellent function in 5 out of 6 patients (83%), while extraplexus neurotisations achieved good or excellent function of the supraspinatus in 30 out of 34 patients (88%). Axillary nerve neurotisation offered good or excellent results (deltoid > M3+ or shoulder abduction > 60°) in 14 patients (58%). Direct neurotisation of the axillary nerve via the motor branch for the long head of the triceps gave shoulder abduction of >110°, as well as external rotation of >30° in 3 out of 5 patients. Combined neurotisation of suprascapular and axillary nerves gave the best outcome achieving shoulder abduction of >60° as well as external rotation of >30°.

Conclusions

Shoulder reanimation should be one of the first priorities in brachial plexus reconstruction. Early neurotisation of the suprascapular, and if possible the axillary nerve offers the best outcome.     

A biomechanical study comparing polyaxial locking screw mechanisms

Objective

Locking plates have become ubiquitous in modern fracture surgery. Recently, manufacturers have developed locking plates with polyaxial screw capabilities in order to optimise screw placement. It has already been demonstrated that inserting uniaxial locking screws off axis results in weaker loads to failure. Our hypothesis was that even implants specifically designed for polyaxial insertion would experience a drop-off in resistance when using non-perpendicular screws.

Methods

Four different types (one monoaxial and three polyaxial locking plates) of readily available small fragment plates were tested. A biomechanical model was developed to test the screws until failure (defined as breakage and rapid loss of >50% force). Screws were inserted at 0, 10 and 15°.

Results

The standard monoaxial locking mechanism sustained saw a 60% reduction in force (332 N vs. 134 N) when screws were inserted cross-threaded at 10°. Two polyaxial systems saw similar significant reductions in force of 45% and 34%, respectively at 15°. A third system utilizing an end cap locking mechanism showed highly variable results with large standard deviations. Polyaxial screws showed on average only limited reduction at 10 degrees of insertion angle.

Conclusion

Newer designs of locking plates have attractive properties to allow more surgical options during fixation. However this freedom comes at the price of reduced force. Our results show that the safe zone for inserting these screws is closer to 20°, rather than the 30° indicated by the manufacturers. Also, the various polyaxial locking mechanisms seem to influence the overall resistance of the screws.