Biomechanical properties of resorbable poly-L-lactide plates and screws: A comparison with traditional systems

J Oral Maxillofac 49:512-516.

Surg

1991

Biomechanical Properties of Resorbable Poly-L-Lactide Plates and Screws: A Comparison JOERG

With Traditional Systems

M. WITTENBERG, MD, DDS,* RALF H. WITTENBERG, AND JOHN A. HIPP, PHDS

MD,t

Poly-L-lactide (PLA) plates and screws were tested in vitro in porcine ribs for uniaxial pullout and four-point bending strength. Results were compared to conventional systems (stainless steel and titanium). The biomechanical testing suggests that PLA screws have properties suitable for fixation of sagittal split osteotomies. Poly-L-lactide plates have indications in areas of low stress and noncompressive load.

Rigid fixation is a commonly accepted technique for treatment of fractures in the craniofacial region. I-3 Screws and plates are also used for fixation of sagittal split osteotomies, maxillary advancements, and many other indications in the maxillofacial field. Most commonly used are rigid fixation systems made of titanium, cobalt alloy, or stainless steel. Possible disadvantages of metallic fixation devices are the atrophic changes of the underlying bone owing to lack of functional stimuli4t5 and the possibility of an allogenic response.6 To overcome these problems, new materials have been developed. Resorbable implants have been produced from materials such as polylactic and polyglycolic polymers. The potential advantages of such devices are twofold: 1) the devices degrade with time thereby reducing stress protection and the accompanying osteoporosis; 2) there is no need for a secondary surgical procedure to remove the resorbable devices. These advantages may facilitate normal bone remodeling at the treatment site and decrease discomfort and cost for the patient.

Indications and uses for resorbable materials have been reported for zygoma fractures,7’8 malleolar fractures,’ and orbital wall and floor reconstruction,‘O~li as well as suspension and fixation wires in facial fractures12 and in closure of oralantral fistulas.” In most studies of poly-L-lactide (PLA) implants, animal models were used and the results were histologically evaluated.‘0*“*13 Recently, Foley et al reported the biomechanical properties of metallic rigid fixation devices in an in vitro mode1,‘4*‘5 but until now no investigations have been undertaken to evaluate the biomechanical properties of the PLA material in bone. The goals of this study were to determine which PLA screw designs show the best biomechanical properties, to compare the PLA devices with the metallic devices, and to determine if PLA meets the biomechanical requirements of a rigid fixation device in the maxillofacial area. These goals were addressed by comparing the pullout and bending strength of screws and plates made from titanium and stainless steel with those made from PLA. Material and Methods

* Resident, Department of Oral and Maxillofacial Surgery, Sinai Hospital, Detroit. t Staff Orthopedic Surgeon, Department of Orthopedic Surgery, Ruhr University, Bochum, Germany. $ Research Assistant, Orthopedic Biomechanics Laboratory, Beth Israel Hospital, Boston. Address correspondence and reprint requests to Dr Joerg M. Wittenberg: Sinai Hospital of Detroit, Department of Oral and Maxillofacial Surgery, 4767 W Outer Dr, Detroit, MI 48235-2899. 0 1991 American geons

Association

of Oral and Maxillofacial

Three separate tests were completed using the porcine rib model. First, the uniaxial pullout strength of screws inserted into intact ribs was determined. Second, ribs were split and fixed with screws. These constructs were tested to failure in four-point bending. Third, transverse osteotomies in ribs fixed with plates were tested in four-point bending. For all tests, fresh porcine rib bone was obtained and the soft tissues were removed. Bones

Sur-

0278-2391/91/4905-0012$3.00/O

512

WITTENBERG

Table 1. Screw Code

ws WT CH

513

ET AL

Metallic Screw Dimensions

1

Screw Type

Drill Hole (mm)

Tap Size

Wurzburg 2.0 mm Wtirzburg 2.7 mm Champy 2.0 mm

1.5 2.0 1.5

Self-tapping 2.7-mm Wurzburg Self-tapping

0

17 nn

0 0

were stored in normal saline and refrigerated for not longer than 5 days. Ko and Hazama have shown that there is no significant change in physical properties of porcine ribs stored in normal saline for up to several months.i6”’ Ribs of equal size were chosen for each of the three tests. For pullout testing (group PT), six different screw designs were studied using seven screws of each design. Screw holes were prepared with a slowly rotating drill. Screw placement was perpendicular to the broad bone surface and through both cortices. Each end of the rib was embedded in polymethylmethacrylate (Tray-Acrylic, Coe Lab, Chicago, IL) in 3-cm-long rectangular aluminum tubes. The placement of the screws was randomized along the ribs to account for normal variation of bone thickness. The metallic screws that were tested in uniaxial pullout are summarized in Table 1, along with the drill hole and tap sizes. Three different PLA screws were developed and tested. Screws were machined from a block of PLA with a density of 1.15 g/cm3 and a molecular weight of approximately 800,000 (Resomer, Boehringer Ingelheim, Germany). Dimensions of the PLA screws are summarized in Table 2. All PLA screw holes were pretapped with the custom-made taps after preparing the guiding holes with a 1.5mm (PLA III) and a 2.0 (PLA II) twist drill. Uniaxial pullout tests of screws were performed on an Instron 1331 servohydraulic mechanical testing system (Instron Corp, Canton, MA). The screw heads were clamped in a custom-made fixture employing a universal joint to relieve any torque of the screws. Screws were pulled from the bone at 0.5 mm/s until failure. The failure force was recorded as the point at which the moment-deflection curve deviated from linearity. The bending stability provided by the six screw Teble 2.

p

-L

I

I

J

I

FIGURE 1. Orthogonal views of the placement osteotomized porcine ribs.

of screws in

types when applied to osteotomized bone was determined using a sagittal split type of osteotomy performed on porcine ribs.‘4*i5 The cancellous segments overlapped along a 1.7-cm interface. Bone screws were placed in a bicortical fashion perpendicular to the surface, with the three screws inserted in a triad fashion approximately 1 cm apart (Fig 1). Six osteotomized ribs were fixed with each of the six screw types summarized in Tables 1 and 2. The ends of the bone segments were then embedded in aluminum tubes using methylmethacrylate and the moment required to deflect the screw/bone constructs to predetermined amounts was measured using a four-point bending configuration (Fig 2). An angular deflection of 20” was arbitrarily designated as ultimate failure. Finally, the stability provided by the three miniplates was investigated. Transverse osteotomies were created in fresh porcine ribs and the segments were fixed in their anatomic relationship with the bone plates. Three types of plates were tested: 1) four-hole Wtirzburg titanium miniplates, 2) fourhole Champy miniplates, and 3) four-hole PLA plates. The PLA plate was injection-molded and supplied through the courtesy of Priv Doz Dr Dr

I

t

lo

01

I

PLA Screw Dimensions

Screw Type

Total Diameter (mm)

Core Diameter (mm)

Thread Depth (mm)

Length (mm)

PLA 1 PLA 2 PLA 3

3.4 3.4 2.6

2.4 2.8 2.0

0.5 0.3 0.3

20 20 20

FIGURE 2.

Four-point bending configuration.

I

514

BIOMECHANICS

Gerlach, Cologne, Germany. Poly-L-lactide III screws were used for the fixation for the PLA plates. All three plates were of similar size and dimension. Six of each plate type were tested. Embedding and mounting of each specimen was performed as previously described and the preparations were tested to failure using four-point bending. In all the experiments, placement and tightening of the screws was consistently performed by one investigator (J.M.W.). For statistical comparison between groups, the Student-Newmann-Keul-test was used, with a significance level of P < .OS.

OF POLY-L-LACTIDE

PLATES AND SCREWS

No significant difference was found in bending moment between the PLA and the metal screws when used for split osteotomy fixation and tested in four-point bending. The bending moment at 4”, 8”, and 12” angular deflection did not show any significant differences for either of these systems (Fig 4). Failure occurred at the osteotomy site in all cases without screw breakage. There were no significant differences in bending moment between the three noncompressive miniplates applied to the osteotomized ribs (Fig 5). Failure generally occurred in the bone at the osteotomy site except in one case where failure occurred at the screw hole of a PLA plate. Discussion

Results Failure forces of 178 to 422 N were recorded during uniaxial pullout tests of bone screws (Fig 3). The three metal screw types failed by buttoning of the bone around the screw (splintering of the bone around the threads), similar to the results of Koranyl.” The pullout force for the metal screws was between 3 18 to 43 1 N and varied significantly between screw types. All PLA III screws failed just below the screw head at 178 ~fr2.0 N. Some PLA I (n = 7) and PLA II (n = 7) screws failed by buttoning of the bone around the screw at 207 f 33.0 N and 270 + 57.6 N, respectively. The remaining PLA I (n = 5) and PLA II (n = 5) screws failed below the head at 274 ? 8.7 N and 326 + 58.3 N, respectively (Fig 3). When comparisons were made between metal and PLA screws, the Wtirzburg 2.0 and Wiirzburg 2.7 withstood significantly (P < .05) higher pullout forces than the Champy and all PLA types. However, the pullout force of the PLA II and of the Champy screws was significantly higher than all of the PLA I and PLA III screws. There were no other significant differences between the screw types.

Rigid fixation of fractures in the maxillofacial area began in Europe around 1970 and has become an essential part of the treatment given by oral and maxillofacial surgeons. Drawbacks for the use of the conventional metallic fixation systems are the occasional need for a second operation to remove the devices and the slight possibility of an allogenic reaction to the material.6 Stress shielding under plates resulting in osteoporotic changes has been observed, suggesting the need to remove the plates.’ This problem has prompted the interest in resorbable materials for reduction of the mandible and orbital floor.21*22 After these initial studies, interest in PLA was lost. Recent reports on the clinical use of PLA in the oral and maxillofacial area have renewed interest. ‘3*23 The PLA material used in this study has been evaluated in animal studies and prototypes have been used in fixation of zygoma fractures in humans.8 Clinical problems, including separation of the screw head and screw body fractures, have been reported (Gerlach KL, personal communication, December 1989). 2.00

,0°< E a 9 (J _I k 3 i a

1.50

t

360

6 P 2

100

F 2

0.50

240

120

d

z

4800 I CH

WT SCREW

PLA

II

PLA

I

PLA

III

TYPE

FIGURE 3. Uniaxial pullout for screws. Error bars are 2 one standard deviation. Screw codes: CH, Champy 2.0 mm; ws, Wtlrzburg 2.0 mm; WT, Wilrzburg 2.7 mm; PLA I,3.4 mm; PLA II, 3.4 mm; PLA III,2.6 mm. Failures at the bone-screw interface are separated from screw failure. 0, Bone failure; tZ, screw failure.

CH

ws

WT

PLA

PLATE

TYPE

I

PLA

/I

PLA

II/

FIGURE 4. Triad screw futation bending moments at three angular deflections. The failure was measured in N-mm at 4”. 8”, and 12” of angular deflection between bone segments. Screw codes: CH, Champy 2.0 mm; WS, Wiirzburg 2.0 mm; WT, Wiirzburg 2.7 mm; PLA I, 3.4 mm; PLA II,3.4 mm; PLA III,2.6 mm. Cl, 4”, 0, 8”, l8( 12”.

WITTENBERG

515

ET AL

4 AtJGLL4R

kg

32

Dey

DFFORMATl01‘1

FIGURE 5. Failure moment of transverse osteotomies fixed by noncompression bone plates at three angular deflections. Error bars are -t one standard deviation. 0, Champy; 0, Wiirzburg; EI. PLA.

Uniaxial pull out tests were performed to evaluate the retentive strength of several screw designs. Torque was eliminated using a custom made pullout jig with a universal-joint. However, the in vitro pullout testing did not address the shear forces and remodelling processes around screws or degradation of PLA screws that would take place in a clinical situation. Foley et al have reported on the uniaxial pullout strength of screws used in the maxillofacial field.14 They determined no difference between pre-tapped and self-tapping screws, but found significantly less retentive strength for Kirschner pins. Foley et al used a monocortical placement; however, we inserted the screws bicortically to simulate clinical situations (eg, orthognathic surgery). The self-tapping WS 2.0 mm and the pretapped WT 2.7 mm screws investigated in this study had the same pullout strength. Only the CH 2.0 and the PLA I, II, III screws had significantly less retentive strength. One major reason for the lower pullout strength of the PLA screws was failure at the screw head or body. Failure did not occur in all cases at the screw-bone interface, as it did with all of the metal screws. This phenomena is best explained by the brittle character of the PLA material and by microfractures created during machining (Gerlach KL, personal communication, 1989). The pullout strength of the PLA II screw was significantly higher than that of the PLA I or III screws and similar to the CH screws. The reason for better retention of the PLA II screws is probably related to a larger core diameter. Poly-L-lactide II screws have a core diameter of 2.8 mm compared to 2.4 and 2.0 mm of the PLA I, PLA III screws; all PLA screws had a 0.7-mm pitch. Thread size varied between 0.3 and 0.5 mm. A PLA thread size of 0.3 mm seems to be optimal, because a deeper (0.5 mm) thread does not improve the retentive strength, which was reported for metal screws with a thread depth of less than 0.25 mm.24 In other in vitro re-

search on metal devices, increased cortical thickness and increased screw external diameter resulted in increased pullout strength.” To account for differences in cortical thickness, screw placement was randomized along the ribs in this study. Internal screw fixation after sagittal split ramus osteotomies is very common and advantages include less relapse tendency,25 shorter hospital stay, better oral hygiene, earlier orthodontic treatment, and minimization of adverse effects on the temporomandibular joint.3 For fixation, three screws are typically used, but site of screw placement is variable. Spiessl prefers two screws above and one below the mandibular canal, while Steinhauuser prefers one above and two below the cana1.26*27Not only the placement, but also how the screws are used is different. Spiessl uses screws as compression or lag screws, which some authors feel is potentially dangerous because of the possibility of condylar displacement and the risk of injury to the mandibular nerve. *’ To reduce these risks, screws are placed bicortically.29 In earlier studies, Foley et al and Ardary et al showed the importance of placement pattern for the rigidity of sagittal split osteotomies.‘5.30 In both of these studies the triad configuration (two screws above, one below the mandibular canal) showed superior rigidity. Therefore, in this study the triad configuration was used and the four-point bending tests were performed to measure rigidity. Four-point bending is superior to the cantilever system used by Foley et al, because it subjects the fixation to a constant bending moment. No significant differences were noted in the screwbone interface for PLA or metal constructs. Variation in screw diameter or in pretapped or selftapped insertion also produced no significant differences in bending moments. The moments required for the initial 4” deflection were the highest, and further deflections of 8” and 12” resulted in relatively lower moments. This is clinically significant because very slight movements between the fragments can influence the healing process.31 The model used here to test the stability provided by various screws and plates after osteotomy in the mandible is a simplification of the clinical situation. Stability after mandibular advancement depends on the degree of advancement or setback and the movement and the position of the osteotomy sites. The osseous contact area and surface area that can be used for screw placement are especially important. Nevertheless it was possible by using this model to demonstrate that PLA screws, used bicortically, may be an adequate alternative for metal screws for fixation of sagittal split osteotomies. The last part of the study tested three different noncompression miniplate systems. The three

516

BIOMECHANICS

miniplate systems investigated are used clinically in midface fractures, after Le Fort osteotomies, and for zygoma fractures. The Champy system is also very popular for fixation of mandibular fractures.32 One problem with these fixation systems is that the relatively thin bones of the midface cannot withstand high forces.3 All plates were placed monocortically and no compression was applied between the fracture fragments. The plate systems tested in four-point bending showed no significant differences between groups; all showed similar properties (Fig 5). In clinical use, the malleability of the material of miniplate systems is important. Titanium systems are known to be readily bendable to the contours of facial bones.* Poly-L-lactide has an elasticity that would make only slight bending possible. To improve malleability, the material may be heated in a hot water bath or by hot electrodes in the area of the bend. However, it is not known if this changes the physical properties of the material. Using the PLA system in mandible fractures is probably one clinical indication, but this will need further preliminary studies. The biomechanical properties of PLA screws and plates are promising. Before using them in humans, however, additional animal experiments should be performed. Injection molding techniques may improve the mechanical strength of PLA devices and could give screws and plates comparable properties to metallic appliances. Acknowledgment We wish to thank the Pacific Dental Research Foundation at the University of the Pacific, San Francisco, for financial support, Walter Lorenz Inc, Jacksonville, FL, for supplying us with the metal devices, and the Boehringer Company, Ingelheim, Germany, for the PLA material.

References 1.Munro IR: The Luhr fixation system for the craniofacial skeleton. Chn Plast Surg 16:41, 1989 2. Marsh JL: The use of the Wiirzburg system to facilitate tixation in facial osteotomies. Clin Plast Surg 16:49, 1989 3. Luhr HG: The significance of condylar position using rigid fixation in orthognathic surgery. Clin Plast Surg 16:147, 1989 4. Paavolainon P, Karahaju E, Slahis P, et al: Effect of rigid plate fixation on structure and mineral content of cortical bone. Clin Orthop 136:287, 1978 5. Tonio AJ, Davidson CL, Klopper PJ, et al: Protection from stress in bone and its effects. J Bone Joint Surg 53B:107, 1976 6. Black J: Biological Performance of Materials. New York, NY, Dekker, 1981 7. Bos RRM, Boering G, Rozema FR, et al: Resorbable Poly(L-Lactide) plates and screws for the fixation of zygomatic fractures. J Oral Maxillofac Surg 45:751, 1987

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PLATES AND SCREWS

8. Gerlach KL: Absorbierbare polymere in der mund und kieferchirurgie. Zahnaerztliche Mitteilung 78:1020, 1988 9. Bostman 0, Vainionpia S, Hiruensalo E, et al: Biodegradable internal fixation for malleolar fractures. J Bone Joint Surg 69B:615, 1987 10. Morian WD, Emerson DC, Stauffer ME, et al: Reconstruction of orbital wall fenestration with polyglactin 910 film. Plast Reconstr Surg 80:769, 1983 11. Rozema FR, Bos RR, Boering G, et al: Bioresorbable poly(L-lactide) implants for blow out fracture repair. An animal study. J Oral Maxillofac Surg 47: 107, 1989 (suppl 1) 12. Comah J, Wallace J: Polydioxanone (PDS), a new material for internal suspension and fixation. Br J Oral Maxillofac Surg 26:250, 1988 13. Gerlach KL: Biologisch abbaubare polymere in der mund-, kiefer- und gesichtschirurgie. Munchen, Germany, Hanser Verlag, 1988 14. Folev WL. Frost DE. Paulin WB. et al: Uniaxial oullout evaluation of internal screw fixation. J Oral Maxiliofacal Surg 47~720, 1989 15. Foley WL, Frost DE, Paulin WB, et al: Internal screw tixation: Comparison of placement pattern and rigidity. J Oral Maxillofacal Sure 47:720. 1989 16. Ko R: The tension test upon compact substance of the long bones of human extremities. J Kyoto Pref Med Univ 53:503, 1953 17. Hazama H: Study on the torsional strength of the compact substance of human beings. J Kyoto Pref Med Univ 60: 167, 1956 18. Koranyl E, Bowman CE, Knecht CD, et al: Holding power of orthopedic screws in bone. Clin Orthop 72:283, 1970 19. Kulkami RK, Pani KC, Neuman C, et al: Polylactic acid for surgical implants. Arch Surg 93:839, 1966 20. Vet-t M, Christel P, Chabot F, et al: Bioresorbable plastic materials for bone surgery, in Hastings D, Ducheyne P (eds): Makromolecular Biomaterials. Boca Raton, FL, CRC Press, 1984 21. Cutright DE, Hunsuck EE, Beasley JD: Fracture reduction using a biodegradable material, polylactic acid. J Oral Surg 29:393, 1971 22. Cutright DE, Hunsuck EE: The repair of fractures of the orbital floor using biodegradable polylactic acid. J Oral Surg 33:28, 1972 23. Brekke JH, Bresner M, Reitman MJ: Polylactic acid surgical dressing material. Postoperative therapy for dental extraction wounds. Can Dent Assoc J 52:599, 1986 24. Ansell RM, Scales JT: A study of some factors which effect the strength of screws and their insertion and holding power in bone. J Biomech 1:279, 1968 25. Paulus GW. Steinhauser EW: A comnarative study of wire osteosynthesis versus bone screws in the treatment of mandibular prognathism. Oral Surg Oral Med Oral Path01 54:2, 1982 26. Spiessl B: Rigid fixation after sagittal split osteotomy of ascending ramus, in Sniessl B (ed): Concepts in Maxillofacial Bone Surgery, New York, NY, Springer Verlag, 1976 27. Steinhauser EW: Bone screws and plates in orthognathic surgery. Int J Oral Surg 11:209, 1982 28. Turvey TA, Hall DJ: Intraoral self-threading screw fixation for sagittal osteotomies. Early experiences. Int J Adult Orthod Orthog Surg 4:243, 1986 29. Niederdellmann H. Shettv V. Collins FJV: Controlled osteosynthesis utilizing the ‘position screw. Int J Adult Orthod Orthog Surg 3: 159, 1986 30. Ardary WC, Tracy DJ, Brownridge GW, et al: Comparative evaluation of screw configuration on the stability of the sagittal split osteotomy. Oral Surg Oral Med Oral Pathol 68: 125, 1989 3 1. Rahn BA: Theoretical considerations in rigid fixation of facial bones. Clin Plast Surg 16:21, 1989 32. Cawood JT: Small plate osteosynthesis of mandibular fractures. Int J Oral Maxillofacial Surg 23:77, 1985