Locked internal fixator: sensitivity of screw/plate stability to the correct insertion angle of the screw

The effect of locked screw angulation on the biomechanical properties of the S.P.S. Free-Block plate

OBJECTIVES

Among the locked internal fixators is one denominated S.P.S. (Synthesis Pengo System) Free-Block, which was designed with a locking ring that allows the screw to be locked and positioned obliquely. Due to the paucity of biomechanical studies on this system, the present work aimed to evaluate the influence of locked screw angulation on the resistance of the S.P.S. Free-Block plate.

METHODS

Forty synthetic bone cylinders with 10 mm fracture gap were used. Forty seven-hole 3.5 mm stainless steel plates (two AO-like dynamic compression holes and five locked holes) were assembled according to the orientation of the locked screws: monocortical screws were positioned at 90º to the long axis of the cylinder (Group 1), and monocortical screws were positioned at 70º to its cylinder long axis (Group 2). In both groups, AO-like dynamic compression hole screws were positioned bicortically and neutrally. For each group, six specimens were tested until failure, three in bending and three in compression, to determine the loads for fatigue testing. Subsequently, for each group, 14 specimens were tested for failure - seven by bending and seven in compression.

RESULTS

No significant failure differences were observed between Groups 1 and 2 under static-loading or fatigue test.

CLINICAL SIGNIFICANCE

In a fracture gap model the orientation of the locked monocortical screws did not show any influence on the mechanical performance of the S.P.S. Free-Block to tests of axial compression and four-point bending.

Advancements in percutaneous fixation for foot and ankle trauma

Operative fixation of foot and ankle trauma can be challenging. Often times, the soft tissue envelope can have extensive damage as a result of the fracture. In these cases, percutaneous fixation may be used. Percutaneous fixation can benefit both soft tissue and osseous healing when used correctly. Many techniques have been described in the literature that may help to preserve blood supply, minimize soft tissue dissection, and restore a functional limb. This article reviews general guidelines for fracture and soft tissue management, osseous healing of fractures, and how certain techniques influence fracture healing. It also illustrates certain techniques for specific fracture reduction.

Locking plate technology and its use in foot and ankle surgery

This article discusses the history of conventional plating, plate biomechanics, and screw function to provide better understanding of osseous physiology and biology using locking plates. The peer-reviewed and non-peer-reviewed literature have been researched to decipher and share the most pertinent information on this topic.

Peri-articular fractures of the knee: an update on current issues

Peri-articular fractures of the knee in the young and elderly pose several management dilemmas. Over the last decade enormous interest has been generated in various fixation modalities, none proving to be an ideal stabilisation method. The problem is compounded by a lack of well-designed studies comparing various treatment options. In this article, the issues surrounding the diagnostic and management strategies of peri-articular fractures of the knee are discussed.

Locked plate fixation of osteoporotic humeral shaft fractures: are two locking screws per segment enough?

OBJECTIVE

The purpose of this study was to compare the biomechanical behavior of using two versus three locking screws per bone segment in a cadaveric humerus fracture gap model.

METHODS

Six matched pairs of elderly osteoporotic fresh-frozen human cadaveric humerii were used. An eight-hole locking compression plate was placed posteriorly on the humeral shaft and secured with either four or six bicortical locking screws. A 5-mm middiaphyseal gap osteotomy was created to simulate a comminuted fracture without bony contact. Specimens were tested in offset axial compression, four-point anteroposterior bending, four-point medial-lateral bending, and torsion. After the initial testing in each of these modalities, the constructs were cyclically loaded in torsion and again tested in the four loading modalities. Lastly, the fixation constructs were then tested to failure in torsion.

RESULTS

There were no significant differences in stiffness between the group fixed with two screws per segment and the group fixed with three screws per segment. The peak torque to failure was higher in the four-screw construct compared with the six-screw construct. The mean torque to failure was 23.5 +/- 3.7 Nm in the construct with two locking screws per segment compared with 20.4 +/- 2.8 Nm in the construct with three locking screws per segment (P = 0.030).

CONCLUSIONS

The addition of a third screw in the locked plate construct did not add to the mechanical stability in axial loading, bending, or torsion. In testing to failure, the addition of a third screw resulted in lower load to failure.

Clinical use of the locking compression plate (LCP) in horses: a retrospective study of 31 cases (2004-2006)

REASONS FOR PERFORMING STUDY

Fracture repair and arthrodesis in horses require rigid fixation in order to achieve early post operative comfort. The locking compression plate (LCP) was designed to allow greater stability and less periosteal disruption than conventional plates.

OBJECTIVE

To describe a series of clinical cases in which the LCP was used for fracture stabilisation or arthrodesis.

METHODS

Medical records, radiographs and follow-up for 31 horses having fracture stabilisation or arthrodesis with the LCP were reviewed. Reasons for treatment included fracture of the olecranon (n = 2), metatarsal III (n = 2), ilial shaft (n = 1), femur (n = 2), radius (n = 2), medial condylar (n = 5) and complex lateral condylar (n = 1); or arthrodesis of the metacarpophalangeal joint (n = 8), proximal interphalangeal joint (n = 8) and carpus (n = 2).

RESULTS

Of the 31 horses, 27 were discharged from the hospital. Complications included incisional infection (32%), implant infection (19%), implant loosening/ breakage (22%), contralateral limb laminitis (16%), colic (3%) and diarrhoea (3%). Follow-up was available on all horses with at least 6 weeks of follow-up ranging up to 25 months. Of the 31 horses, 25 are sound for intended purpose, one is lame and 5 were subjected to euthanasia for complications associated with the original injury.

CONCLUSIONS

The LCP is an acceptable means of fracture stabilisation and arthrodesis in the horse.

POTENTIAL RELEVANCE

The LCP should be considered for difficult fracture configurations and arthrodeses, especially when increased stability of the fixation is needed.

Biomechanical comparison of polyaxial-type locking plates and a fixed-angle locking plate for internal fixation of distal femur fractures

OBJECTIVES

To test the stability to axial loading of 2 new polyaxial locking screw-plate designs and analyze different angles of screw insertion. The noncontact bridging (NCB) polyaxial locking plate (Zimmer) and the POLYAX plate (DePuy) were compared with a fixed-angle less invasive stabilization system (LISS; Synthes).

METHODS

Twenty-five synthetic femurs were divided into 5 groups and assigned fixation with the LISS plate (group I), POLYAX plate (groups IIA and IIB), or NCB plate (groups IIIA and IIIB). The polyaxial constructs were divided into parallel and crossed distal condylar screw configurations. Each construct was tested under axial loading and stressed to failure at a displacement rate of 5 mm/min with a preload of 100 N. Outcome measurements included stiffness, load to failure, peak force, and mode of failure.

RESULTS

All LISS and POLYAX constructs failed by plastic deformation of the plate, whereas 9 of 10 NCB constructs failed by an intra-articular lateral condyle fracture. No failures occurred at the screw-plate interface in either polyaxial constructs. Load to failure of the LISS was 33% greater than the parallel POLYAX (P < 0.01) and 24% greater than the crossed POLYAX (P < 0.01). Load to failure of NCB (parallel and crossed) were 24% greater than the parallel POLYAX (P < 0.01 and P < 0.01, respectively) and 15% greater than the crossed POLYAX (P < 0.01 and P = 0.02, respectively). The POLYAX also had significantly lower stiffness and peak force compared with the LISS and NCB. There was no difference between the LISS and NCB with regard to stiffness, load to failure, and peak force. Parallel and crossed polyaxial constructs showed no difference in stiffness or failure loads.

CONCLUSIONS

There were no failures of either polyaxial screw-plate interface despite large forces and screw angle did not affect the overall strength of these constructs, supporting the biomechanical soundness of both polyaxial device designs under axial loading. However, the POLYAX supported smaller loads compared with the LISS and NCB while under axial loading. In addition, the mode of failure of the NCB plate, creating an intra-articular fracture propagating from the distal posterior screw hole, may be of some concern. Additional testing is needed to determine the clinical importance of the demonstrated differences among these plate designs.

Analysis of early failure of the locking compression plate in osteoporotic proximal humerus fractures

BACKGROUND

Although there has been continuous evolution in the management of fracture fixation, treatment for osteoporotic proximal humerus fractures is still challenging to trauma surgeons. The purpose of this study was to report early failure of the locking compression plate (LCP) in the treatment of osteoporotic proximal humerus fracture and characterize the mode of failure.

METHODS

Nine patients, older than 65 years, underwent internal fixation with the use of a locking compression plate and had early failure within 4 weeks postoperatively. According to Neer's classification, five were included in a two-part surgical neck fracture, three in a three-part fracture, and one in a four-part fracture.

RESULTS

All failures occurred with back-out of the plate-screw construct, leading to varus displacement in eight patients and plate breakage in one. Revision surgery was performed in six patients using replating and tension band wiring with a bone graft, and three patients underwent hemiarthroplasty. The average UCLA score was 25 points for the hemiarthroplasty group and 30 points for the reconstruction group.

CONCLUSIONS

Early postoperative failure of the LCP developed within 4 weeks with a presentation of en bloc back-out of the plate-screw construct and plate breakage. Possible risk factors included malreduction, loss of medial support, and negligence of tension band sutures on the tuberosities.

Cement as a locking mechanism for screw heads in acetabular revision shells - a biomechanical analysis

In acetabular revisions, polyethylene (PE) liners are often cemented into metal shells on top of acetabular screw heads. This study investigates the possibility of using this technique to obtain fixed-angle acetabular screws Eth a concept that has not yet been reported in the literature. Two groups of screws (n=8) were inserted into Trabecular Metal revision shells (Zimmer), into which PE liners were then cemented. Screws in Group 1, inserted in the shell's pre-fabricated holes, were countersunk, whereas screws in Group 2 were inserted in custom-drilled holes that make their heads protrude into, and interdigitate with, the overlying cement mantle. Perpendicular loading was then applied to the screw shafts both statically to failure and cyclically. A greater stiffness was observed for the protruding screws upon static loading; and while the countersunk screws all failed at the screw-cement junction (53.44 + or - 8.33 N), the protruding screws all failed at the screw shaft (1049.79 + or - 32.12 N) a 20-fold difference (p< 0.05). Under cyclic loading, only the protruding screw head specimen did not fail, undergoing an overall displacement within the limits of osseointegration.These results support the hypothesis that the protrusion of an acetabular screw head into an overlying cement mantle significantly increases its angular stability. Provided other variables are favorable, this locking effect may increase the initial stability of the whole implant, thus improving the ultimate success of complex acetabular revisions.

Fractures of the distal tibia treated with polyaxial locking plating

We evaluated the healing rate, complications, and functional outcomes in 32 adult patients with very short metaphyseal fragments in fractures of the distal tibia treated with a polyaxial locking system. The average distance from the distal extent of the fracture to the tibial plafond was 11 mm. All fractures healed and the average time to union was 14 weeks. Six patients (19%) reported occasional local disturbance over the medial malleolus. There were two cases of postoperative superficial infections and evidence of delayed wound healing. Using the American Orthopaedic Foot and Ankle Society ankle score, the average functional score was 87.3 points (of 100 total possible points). Our results show the polyaxial locking plates, which offer more fixation versatility, may be a reasonable treatment option for distal tibia fractures with very short metaphyseal segments.

Technical difficulties of removal of locking screw after locking compression plating

INTRODUCTION

As there are few reports on the difficulties of removing the locking compression plate (LCP), we prospectively investigated the incidence and difficulties in 58 patients in whom various types of LCPs were taken.

METHODS

From January 2004 to December 2007, we have removed 159 5.0-self tapping locking screws and 279 3.5- self tapping locking screws. All of the operations were performed by experienced trauma surgeons. All of the screws were inserted with the use of torque limiting attachment according to the manufacturer's recommendation. During the same period of time we have removed 198 AO-3.5 cortical and 4.0 cancellous screws from various sites.

RESULTS

All of 159 5.0-self tapping locking screws were removed without difficulties. A total of 24 out of 279 3.5- self tapping locking screws were removed with many difficulties due to the stripping of the hexagonal recess. The use of conical extraction screw which was developed especially for the removal of stripped locking screws was successful in only six screws. We have removed plates by cutting the plate with metal cutting saw. We describe useful technical trick to remove the plate when there is only one screw left stripped. Compared to the locking screws, only one of 198 3.5-cortical screws was stripped.

CONCLUSION

Care should be taken at the time of removal of the locking compression plate, especially for the 3.5-locking screws.

Locked plating in orthopaedic trauma: a clinical update

Locked plating for fracture fixation has enjoyed widespread popularity despite a paucity of published data on outcomes. Anatomically precontoured locked plates that allow fixation in various anatomic regions are widely available. New technologies incorporate subchondral support locking pegs, polyaxial bushings, and locking washers to improve intraoperative versatility. However, limited data are available on the efficacy of these new implants. The clinical performance of locked plates generally has been good. However, several unique complications have been noted, such as difficulty with implant removal, malalignment, fracture distraction, and loss of diaphyseal fixation, especially with percutaneous techniques and unicortical screws. The expense of locked plate constructs is a concern. This technology typically costs three times more than similar unlocked constructs. Locked constructs should be reserved for problematic fractures that have demonstrated poor outcomes with unlocked constructs.

Fixation of HA-coated unicortical locking screws in a sheep gap model: a comparative biomechanical study

OBJECTIVES

To evaluate whether fixing a locking compression plate (LCP) with hydroxyapatite (HA)-coated screws provides improved biomechanical fixation and healing compared with standard screws under loaded conditions.

METHODS

Ten sheep were randomly divided into 2 groups. A resection osteotomy of 8 mm was performed in the sheep's right tibiae. Five tibiae were fixed with a 9-hole LCP and 8 standard unicortical locking screws (Group Non-HA), and 5 tibiae were fixed with a 9-hole LCP and 8 HA-coated unicortical locking screws (Group HA). All screws were implanted at the same insertion torque of 4000 Nmm. Three months after surgery, all the sheep were euthanized. Bone segments after screw removal were randomly chosen from each group for histologic analysis (Group Non-HA=5, Group HA=5).

RESULTS

Mean screw extraction torque was 438+/-288 Nmm in Group Non-HA (n=40) and 2317+/-657 Nmm in Group HA (n=40) (P<0.0005). The tibial torque resistance of the resected tibiae was 24+/-8 Nm in Group Non-HA (n=5) and 31+/-3 in Group HA (n=5) (P=0.045). In Group Non-HA, histology showed bone resorption and fibrous tissue encapsulation in all the samples, but this was not found in any of the Group HA samples.

CONCLUSIONS

This study shows that an LCP with HA-coated screws provides improved biomechanical fixation than an LCP with similar standard screws as shown by a 5-fold greater screw extraction torque (P<0.0005). Furthermore, the higher tibial torque resistance is potentially beneficial for improved gap healing, as shown by higher tibial torque resistance.

A review of locking compression plate biomechanics and their advantages as internal fixators in fracture healing

Metallic implants are often involved in the open reduction and internal fixation of fractures. Open reduction and internal fixation is commonly used in cases of trauma when the bone cannot be healed using external methods such as casting. The locking compression plate combines the conventional screw hole, which uses non-locking screws, with a locking screw hole, which uses locking head screws. This allows for more versatility in the application of the plate. There are many factors which affect the functionality of the plate (e.g., screw placement, screw choice, length of plate, distance from bone, etc.). This paper presents a review of the literature related to the biomechanics of locking compression plates and their use as internal fixators in fracture healing. Furthermore, this paper also addresses the materials used for locking compression plates and their mechanical behavior, parameters that control the overall success, as well as inherent bone quality results.

Locking plate technology and its role in osteoporotic fractures

Osteoporosis is a public health issue that leads to increased morbidity and dependency in an increasing population. Fractures in osteoporotic bone have been a contraindication to internal fixation due to their past poor results. Various treatment methods and innovations have been attempted that have met with some success. This paper reviews one such innovation, locking-plate osteosynthesis, looking at its technology and success and its role in osteoporotic patients. As this is an evolving technology, surgeon's experience with these plates and screws are in advance of clinical studies and therefore the surgeons should proceed with caution when choosing locking plate technology in the treatment of osteoporotic fractures.

Results of polyaxial locked-plate fixation of periarticular fractures of the knee

BACKGROUND

Locked-plate fracture-fixation techniques and designs continue to evolve. Polyaxial locking plates that allow screw angulation and end-point locking have become available; however, there are no clinical data documenting their strength and efficacy, to our knowledge. The purpose of this study was to evaluate the clinical performance of a variable-axis locking plate in a multicenter series of periarticular fractures about the knee.

METHODS

Between 2003 and 2005, fifty-four patients with a total of fifty-six fractures were treated with a polyaxial locked-plate fixation system (DePuy, Warsaw, Indiana). There were twenty male patients and thirty-four female patients with a mean age of fifty-seven years. There were twenty-five distal femoral fractures and thirty-one proximal tibial fractures. Twelve of the fractures were open. Clinical and radiographic data, including changes in alignment, hardware breakage, or other mechanical complications of the device, were retrospectively reviewed. Function was assessed with use of the Knee Society scores. One patient with a bilateral fracture died less than three months postoperatively, and two patients were lost to follow-up prior to union. Fifty-two fractures in fifty-one patients were followed to union or for a minimum of six months; the mean duration of follow-up was nine months (range, six to twenty-five months).

RESULTS

Forty-nine (94%) of the fifty-two fractures united. There were no mechanical complications. Most importantly, there was no evidence of varus collapse as a result of polyaxial screw failure. There were three deep infections and one aseptic nonunion. No plate fractured, and no screw cut out.

CONCLUSIONS

The variable-axis locking plates performed well, with a high rate of fracture union and no evidence of varus collapse due to failure of the polyaxial screw fixation, in a series of complex fractures about the knee. Complication rates were similar to those for historical controls treated with fixed-trajectory locking plates. Polyaxial locking plates offer more fixation versatility without an apparent increase in mechanical complications or loss of reduction.

Humeral shaft split fracture around proximal humeral locking plates: a report of two cases

Locking plates increasingly are being used in the treatment of osteoporotic fractures. Such devices provide multiple fixed- angle anchorage points and improve fracture fixation stability in weak bone. We report two cases of early proximal humeral fracture fixation failure in osteoporotic bone by humeral shaft "fissure" or split fracture after open reduction and internal fixation with locking proximal humeral plates and screws. Each patient failed early in the postoperative period and was revised to fixation with compression plating techniques, with uneventful union.

Polyaxially-locked plate screws increase stability of fracture fixation in an experimental model of calcaneal fracture

Different calcaneal plates with locked screws were compared in an experimental model of a calcaneal fracture. Four plate models were tested, three with uniaxially-locked screws (Synthes, Newdeal, Darco), and one with polyaxially-locked screws (90° ± 15°) (Rimbus). Synthetic calcanei were osteotomised to create a fracture model and then fixed with the plates and screws. Seven specimens for each plate model were subjected to cyclic loading (preload 20 N, 1000 cycles at 800 N, 0.75 mm/s), and load to failure (0.75 mm/s).

During cyclic loading, the plate with polyaxially-locked screws (Rimbus) showed significantly lower displacement in the primary loading direction than the plates with uniaxially-locked screws (mean values of maximum displacement during cyclic loading: Rimbus, 3.13 mm (sd 0.68); Synthes, 3.46 mm (sd 1.25); Darco, 4.48 mm (sd 3.17); Newdeal, 5.02 mm (sd 3.79); one-way analysis of variance, p < 0.001).

The increased stability of a plate with polyaxially-locked screws demonstrated during cyclic loading compared with plates with uniaxially-locked screws may be beneficial for clinical use.