technical report - inion freedomscrew™ - activize - pre...

Inion Oy August 19th, 2013

Company confidential – Not for official use

Technical Report - Inion FreedomScrew™

Authors: Petteri Väänänen, Product Development Engineer Tuomas Heinonen, Mechanical Design and Testing Engineer Timo Pohjonen, R&D Director Timo Allinniemi, Business Unit Director - Orthopedic

Inion Oy Technical Report /14 Inion FreedomScrew™


Table of contents 1 Background and purpose ..................................................................................................................... 3 2 Products under investigation ................................................................................................................ 3

2.1 Raw materials, manufacturing and sterilization ........................................................................... 3 2.2 Intended use ................................................................................................................................ 3 2.3 Clinical safety ............................................................................................................................... 3 2.4 Use with Inion FreedomPlate

TM ................................................................................................... 4

2.5 Screw head cutting with temperature cautery ............................................................................. 4 3 Testing methods ................................................................................................................................... 5

3.1 Mechanical properties of Inion FreedomScrew™........................................................................ 5 3.2 Fixation properties of Inion FreedomScrew™ ............................................................................. 5 3.3 Fixation properties of Inion FreedomScrew™ with the Inion FreedomPlate™ ........................... 6 3.4 Degradation properties of Inion FreedomScrew™ ...................................................................... 8

4 Results .................................................................................................................................................. 9 4.1 Mechanical properties of Inion FreedomScrew™........................................................................ 9 4.2 Fixation properties of Inion FreedomScrew™ ........................................................................... 10 4.3 Fixation properties of Inion FreedomScrew™ with the Inion FreedomPlate™ ......................... 11 4.4 Degradation properties of Inion FreedomScrew™ .................................................................... 12

5 Conclusion .......................................................................................................................................... 12 6 Discussion .......................................................................................................................................... 13 7 References ......................................................................................................................................... 13

Reference standards and guidance documents

Standard Description

ASTM F1717 Standard test methods for static and fatigue for spinal implant constructs in a corpectomy model

ASTM F2502 Standard specification and test method for absorbable screws and screws for internal fixation implants

BS 2782 Methods of testing plastics - Part 3: Mechanical properties - Method 340B: Determination of shear strength of sheet material

ISO 13781 Poly(L-lactide) resins and fabricated forms for surgical implants - In vitro degradation testing

ISO 15814 Implants for surgery - Copolymers and blends based on polylactide - In vitro degradation testing

FDA Guidance (draft)

Guidance document for testing biodegradable polymer implant devices



1 Background and purpose This technical report summarizes the outcomes and results of the testing conducted for the Inion FreedomScrew™ products to verify and ensure their mechanical, fixation and degradation properties. Tests were conducted and their results were compared to those of the currently approved and clinically used products with same intended use, i.e., Inion OTPS™ Screws [3-6,11,13-14,19-23,27,30,33].

2 Products under investigation Inion FreedomScrew™ product portfolio contains a wide set of screws for orthopaedic indications. Inion FreedomScrew™ products are provided as fully and partially threaded (i.e., lag), and as solid and cannulated versions. Sizes of the Inion FreedomScrew™ cover diameters from 2.0 mm to 4.5 mm and lengths from 10 mm to 90 mm.

2.1 Raw materials, manufacturing and sterilization

The Inion FreedomScrew™ products are made of degradable co-polymer composed of L-lactic acid and D-lactic acid. This material composition provided by Inion Oy has a long and successful history of safe and efficient medical use in several clinical applications in the field of orthopaedic surgery [3-6,11,13-14,19-23,27,30,33] and it degrades in-vivo by hydrolysis into alpha-hydroxy acids that are metabolised by the body. Inion FreedomScrew™ products are manufactured utilizing self-reinforcement process which provides significant improvement in material strength of the screws when compared to non-self-reinforced screws made out of same material. Inion FreedomScrew™products are sterilized with gamma irradiation using a dose of 25-32 kGy.

2.2 Intended use

The Inion FreedomScrew™ products are intended for maintenance of alignment and fixation of bone fractures, comminuted fractures, osteotomies, arthrodeses or bone grafts (i.e., autografts or allografts) in the presence of appropriate additional immobilization (e.g., rigid fixation implants, cast or brace). In addition, the Inion FreedomScrew™ Ø3.5/4.0/4.5 mm products are specifically intended for use in following indications: A: general indications: maintenance of reduction and fixation of cancellous bone fractures, osteotomies or arthrodeses of the upper extremity, ankle and foot in the presence of appropriate brace and/or immobilization, and B: specific indications: fractures and osteotomies of the malleoli, and ankle fractures. Inion FreedomScrew™ products can also be used in conjunction with appropriate Inion® biodegrdable plates, such as the Inion FreedomPlate™. Inion FreedoPlate™ is a new type of a biodegradable plating system recently designed and introduced by Inion Oy.

2.3 Clinical safety

Previously Eitenmüller et al. (1996) reported postoperative problems related to degradation of implants such as tissue reactions due to bulky implants and protruding screw heads when using biodegradable plates and screws in fixation of ankle fractures [8]. The composition of biodegradable materials have since been developed to be better tolerated by the human body thus causing less complications. However, even though good clinical results in treatment of ankle fractures have been reported with these biodegradable implants with a novel material composition [11,13-14,27] postoperative issues due to protruding and degrading screw heads in ankle fracture cases are still reported [13]. It has also been suggested that possibility to remove screw heads and create low-profile fixation might significantly



decrease incidence of these issues [13]. Considering those issues Inion FreedomPlate™ was developed and designed.

2.4 Use with Inion FreedomPlateTM

Inion FreedomPlate™ has some essential differences compared to conventional metal and biodegradable plates. After heating in a warm water bath, the plate can be cut to the desired size and shape and easily contoured to match with the contours of the bone surface. The Inion FreedomPlate™ does not have readymade screw holes, only several pilot holes allowing fluid to flow through the plate and also having the freedom to drill holes through the desired pilot holes in the desired direction (angulation) for optimal screw position in relation to fracture line(s) and bone quantity/quality. The most significant difference to conventional plate is that to achieve a low-profile seating of the screw, the screw heads can be cut off along the plate surface after screw insertion.

Figure 1: Contoured Inion FreedomPlate™ and screws with heads removed.

2.5 Screw head cutting with temperature cautery

Contrary to screw head removal with pliers as is necessary with older generation screws, the Inion FreedomScrew™ heads, when used in conjunction with the Inion FreedomPlate™, can be removed by using the temperature cautery device. Due to self-reinforced technology used in the production of the Inion FreedomScrew™ products, the material of the screws expands when heated (see Figure 2). therefore when the screw head removal is performed with the temperature cautery device approximately 1-2 mm above the plate surface, not only is the screw head removed to create low-profile plate-screw interface but also a secondary screw head is created and the screw shaft expands within the screw hole of the plate making plate-screw interface more firm than when cutting pliers is used.

Figure 2: Expansion of material of the Inion FreedomScrew™ products when heated with temperature cautery device.



3 Testing methods The mechanical, fixation and degradation properties of the Inion FreedomScrew™ products were determined and verified by the test methods as described in the following sections. In addition to conventional mechanical and fixation property tests the fixation properties of the Inion FreedomScrew™ products with the Inion FreedomPlate™ were also ensured and verified. Testing was generally conducted by following the guidelines of the FDA guidance documents, using ASTM F2502, ISO 13781 and ISO 15814 standards. The intact test samples were placed in separate closed laboratory test tubes filled with phosphate buffer solution and stored at 37±1 ºC for 24 h (and over other appropriate time periods) prior to testing. All tests were conducted in water at 37±1 ºC. Testing was conducted by using Zwick Z020/TH2A universal materials testing machine (Zwick GmbH, Germany) with specially designed test jigs and fixtures.

3.1 Mechanical properties of Inion FreedomScrew™

Mechanical properties of the Inion FreedomScrew™ products were determined by two-sided shear tests following the guidelines of the BS 2782. In shear testing the screw shaft was loaded with constant speed of 10 mm/min in the shear test fixture (Figure 3) until failure of the screw.

Figure 3: Shear test setup and shear test fixture for the screws on the left. Schematic illustration of the shear test on the right. Note that actual testing was performed in water.

3.2 Fixation properties of Inion FreedomScrew™

Fixation properties of the Inion FreedomScrew™ products were determined by pull-out tests from bone substitute material (solid rigid polyurethane foam blocks with density of 20 pcf; Sawbone, Pacific Research Laboratories Inc., US) and from rigid material (polyethylene; Vink Finland Oy, Finland). Testing from bone substitute material provides evidence about fixation properties of the screw thread design which replicates the clinical setting. The testing from rigid material allows for the screws mechanical properties to be evaluated under high tension stress without the normal failure mechanism caused by bone substitute. The use of these carrier materials has been recommended by medical device testing standards such as ASTM F1839 and ASTM F2502. Furthermore, these materials have been widely used as carrier material in testing of fixation screws and are known to tolerate hydrolytic in vitro conditions without weakening or undergoing other changes in its properties [25,28,34-35]. Therefore these carrier materials also enable detailed evaluation of changes in properties of the implants under investigation, (not changes in carrier material) and improves measuring accuracy.



To prepare pull-out test specimens an appropriate hole for the screw was drilled and tapped into the test blocks perpendicular to the surface of the block. The screw was then inserted in the block with the metallic test jig (Figure 4). In pullout test the screw was fixed to the test machine via foam block and metallic test jig (Figure 4). The screw (i.e. metallic test jig) was pulled upwards from the head of the screw in parallel to long axis of the screw with constant speed of 5 mm/min until failure of the screw or fixation.

Figure 4: Test set-up for the pull-out test. Note that actual testing was performed in water.

3.3 Fixation properties of Inion FreedomScrew™ with the Inion FreedomPlate™

Fixation properties of the screws with the Inion FreedomPlate™ were tested in specific plate-screw fixation tests. In this test PE-blocks were used as carrier material. The use of this carrier material has been recommended by medical device testing standards such as ASTM F1717. Furthermore, this material has also been previously used as carrier material in testing of fixation screws and known to tolerate hydrolytic in-vitro conditions without weakening or undergoing other changes in their material properties [34]. Therefore, use of this carrier material ensures that test results reflect fixation properties of implants rather than those of the carrier material, and also improves measuring accuracy. To prepare plate-screw fixation test specimens an appropriate hole for the screw was drilled and tapped through the most middle pilot hole of the test sample plate specimen and simultaneously into the PE-block perpendicular to plate and block. Then screw was inserted into the drilled and tapped hole and used to fix the test sample plate together with the PE-block and the metallic test jig (Figure 5).



Figure 5: The schematic drawing of test specimen for the plate-screw fixation test. The loading direction during testing is shown with a red arrow. After screw insertions, to create low-profile and so called “headless screw” fixation, the screw heads of all Inion FreedomScrew™ test samples were removed according to surgical instructions approximately 1-2 mm above the plate surface by using the temperature cautery device (Figure 6). Comparison samples (i.e., Inion FreedomPlate™ fixed with inion OTPS™ Screw) were prepared similarly but screw heads were removed along surface of the plate by cutting pliers as instructed by the manufacturer (Figure 6). Usage of a single screw fixation set-up provides the worst case situation for plate-screw fixation and improves measuring accuracy when all loading in this test set-up was carried by a single screw (and plate-screw interface) only.

Figure 6: Removing of the screw head by the temperature cautery device (on the left) and by the cutting pliers (on the right). Prior to testing the specimens (i.e., screw fixed with the plate, PE-block and metallic test jig) were placed in separate closed laboratory test tubes filled with phosphate buffer solution (pH 7.4±0.2) and stored at 37±1 ºC over 24 h.



In plate screw pullout tests the specimen was fixed to the test machine by the PE-block and metallic test jig (Figure 7). The plate (i.e. metallic test jig) was pulled upwards from the test jig in parallel to long axis of the screw with constant speed of 5 mm/min until failure of the screw or fixation.

Figure 7: Test set-up for the plate-screw fixation test. Note that actual testing was performed in water.

3.4 Degradation properties of Inion FreedomScrew™

The hydrolytic in vitro degradation period was carried out to determine the change in mechanical properties and verify the sufficiency of the mechanical stability over healing period of the Inion FreedomScrew™ products, and to ensure degradation of the products. During in vitro degradation period, changes in mechanical properties of the Inion FreedomScrew™ were determined by shear test (as described in section 3.1 above).



4 Results

The following sections 4.1-4.4 summarize the results of the tests conducted with the Inion FreedomScrew™ products and showing their comparison to the Inion OTPS™ Screws.

4.1 Mechanical properties of Inion FreedomScrew™

Chart 1 below shows the initial mechanical properties of the Inion FreedomScrew™ products, i.e., the results of the shear tests in comparison to those of the comparable Inion OTPS™ Screws.

*significantly higher value (p<0.05) in comparison to comparable Inion OTPS™ Screw

Chart 1: Initial mechanical properties.



4.2 Fixation properties of Inion FreedomScrew™

Chart 2 below shows the pull-out capacity of the Inion FreedomScrew™ products from rigid material, i.e., the improvement in mechanical properties in comparison to those of the comparable Inion OTPS™ Screws.


Chart 2: Pull-out capacity from rigid material.

Chart 3 below shows the pull-out capacity of the Inion FreedomScrew™ products from bone substitute material, i.e., the improvement in screw designs in comparison to those of the comparable Inion OTPS™ Screws.


Chart 3: Pull-out capacity from bone substitute material.



4.3 Fixation properties of Inion FreedomScrew™ with the Inion FreedomPlate™

Chart 4 below shows the fixation capacity of the Inion FreedomScrew™ products with the Inion FreedomPlate™ with removed screw heads, i.e., the results of the plate-screw fixation tests in comparison to those of the Inion OTPS™ Screw.

*significantly higher value (p<0.05) in comparison to Inion OTPS™ Screw

Chart 4: Fixation capacity of the screws with the Inion FreedomPlate™ with removed screw heads.



4.4 Degradation properties of Inion FreedomScrew™

Chart 5 below shows the degradation of mechanical strength of the fixation Inion FreedomScrew™ products during in vitro degradation period and comparison to that of the Inion OTPS™ Screws.

*significantly lower value (p<0.05) in comparison to the initial value with same product

Chart 5: Degradation of mechanical strength of the screws during in vitro degradation period.

5 Conclusion Inion FreedomScrew™ products achieved better results in comparison to the comparable Inion OTPS™ Screws in all tests conducted and therefore Inion FreedomScrew™ products have superior mechanical and fixation properties. In addition, in the case of the Inion FreedomScrew™, the screw head removal concept with the temperature cautery device provides enhanced fixation properties with the Inion FreedomPlate™ compared with the screw head removal by cutting pliers with the Inion OTPS™ Screw and Inion FreedomPlate™. In addition to the fact that the Inion FreedomScrew™ products have superior initial mechanical properties, they also retain their properties longer during the degradation period when compared to the Inion OTPS™ Screws.

*

*



6 Discussion Unlike metal implants, there are no standards defining the specific strength requirements for polymer implants. Furthermore, polymer implants are used in a different way than metal implants, i.e. after surgical treatment of orthopaedic fractures, the patient is not allowed to bear weight and is immobilized with plaster cast and splints. The use of crutches, etc., are recommended 4-8 weeks postoperatively [7,9-10,12-13,27,31-32]. Therefore, the most important purpose of the implant is actually to provide and maintain optimal fracture reduction (keep the fracture ends together) during fracture healing while the cast takes care of most of the immobilization and stabilization. In theory, initially, 100 % of the strength of the repair site is provided by implant used. Over the primary healing period the loading at the repair site is shared between the implant and partially healed tissues, union and other physiological structures. Ideally, the retention of strength properties of the implant would mirror the strength of biological structures over an appropriate healing period so that substantial strength on repaired site would exist over healing [1-2,26]. Depending on different factors such as severity and type of fracture, clinical approach, healing criteria used, etc. average healing time needed for sufficient preliminary union in orthopedic cases have been reported to be approximately 6-9 weeks [13,16,18,24]. This time window is also in line with healing time required and reported for fractures of other areas [15,17]. Furthermore, as general fundamental rule, about 6-8 weeks is required for good primary healing and after 6-8 weeks approximately 70 % of healing process, such as biological union, is completed [1-2,29]. In addition, after surgical treatment of more demanding orthopaedic fractures, the patient is not allowed to bear weight and immobilizations, i.e., plaster cast, splint, use of crutches, etc., are required 4-8 weeks postoperatively [7,9-10,12-13,27,31-32]. Therefore, the most important purpose of the implant is actually to provide and maintain optimal fracture reduction (keep the fracture ends together) during fracture healing while e.g. the cast takes care of most of the immobilization and stabilization.

7 References

1. An YH, et al. Fixation of osteotomies using bioabsorbable screws in the canine femur. Clin Orthop Relat Res 1998;355:300-11.

2. An YH, et al. Pre-clinical in vivo evaluation of orthopaedic bioabsorbable devices. Biomaterials 2000;21:2635-52.

3. Atoun E, et al. Arthroscopically assisted internal fixation of the symptomatic unstable os acromiale with absorbable screws. J Shoulder Elbow Surg 2012;e-pub ahead of print.

4. Campbell N, et al. Surgical stabilization of rib fractures using Inion OTPS wraps - Techniques and quality of life follow-up. J Trauma 2009;67:596–601.

5. Dhillon MS, et al. Preliminary experience with biodegradable implants for fracture fixation. Indian J Orthop 2008;42:319-22.

6. Edmonds EW. Use of an absorbable plate in the management of a clavicle fracture in an adolescent. Am J Orthop (Belle Mead NJ) 2012;41:29-32.

7. Egol KA, et al. Distal radial fractures in the elderly: operative compared with nonoperative treatment. J Bone Joint Surg Am 2010;92:1851-7.

8. Eitenmüller J, et al. Surgical treatment of ankle joint fractures with biodegradable screws and plates of poly-l-lactide. Chirurg 1996;67:413-8.

9. Figl M, et al. Unstable distal radius fractures in the elderly patient - volar fixed-angle plate osteosynthesis prevents secondary loss of reduction. J Trauma 2010;68:992-8.

10. Hovis WD, et al. Treatment of syndesmotic disruptions of the ankle with bioabsorbable screw fixation. J Bone Joint Surg Am 2002;84:26-31.

11. Kainonen T, et al. Healing of ankle fractures: comparison of biodegradable and metal plate and screws. Annual Meeting of American Academy of Orthopaedic Surgeons, paper no. 504, San Fransisco, US, 2008.

12. Korkala O, et al. Biodegradable screw fixation of thesyndesmosis together with metallic osteosynthesis. Preliminary experience of 7 ankles. Ann Chir Gynaecol 1999;88:295-7.

http://journals.lww.com/corr/Abstract/1998/10000/Fixation_of_Osteotomies_Using_Bioabsorbable_Screws.32.aspx



13. Kukk A, et al. A retrospective follow-up of ankle fracture patients treated with a biodegradable plate and screws. Foot Ankle Surg 2009;15:192-7.

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15. Laughlin RM, et al. Resorbable plates for the fixation of mandibular fractures: a prospective study. J Oral Maxillofac Surg 2007;65:89-96.

16. Lehtonen H, et al. Use of a cast compared with a functional ankle brace after operative treatment of an ankle fracture. A prospective, randomized study. J Bone Joint Surg Am 2003;85:205–11.

17. Leonhardt H, et al. INION compared with titanium osteosynthesis: a prospective investigation of the treatment of mandibular fractures. Br J Oral Maxillofac Surg 2008;46:631-4.

18. Mahajan V, et al. Fractures of the proximal fifth metatarsal: percutaneous bicortical fixation. Clin Orthop Surg 2011;3:140-6.

19. Marasco S, et al. Pilot study of operative fixation of fractured ribs in patients with flail chest. ANZ J Surg 2009;79:804-8.

20. Marasco SF, et al. Mode of failure of rib fixation with absorbable plates: a clinical and numerical modeling study. J Trauma 2010;68:1225-33.

21. Mavrogenis AF, et al. Early experience with biodegradable implants in pediatric patients. Clin Orthop Relat Res 2009;467:1591-8.

22. Noh JH, et al. Outcomes of operative treatment of unstable ankle fractures: a comparison of metallic and biodegradable implants. J Bone Joint Surg Am 2012;94:e166.

23. Novak E, et al. Versatility of bioabsorbable osteosynthesis materials in hand surgery. European Journal of Trauma 2006;1:S133.

24. Orbay JL, et al. Volar fixation for dorsally displaced fractures of the distal radius: a preliminary report. J Hand Surg Am 2002;27:205-15.

25. Pietrzak WS, et al. A bioabsorbable fixation implant for use in proximal interphalangeal joint (hammer toe) arthrodesis: biomechanical testing in a synthetic bone substrate. J Foot Ankle Surg 2006;45:288-94.

26. Pietrzak WS, et al. Bioresorbable implants - Practical considerations. Bone 1996;19:109S-119S. 27. Rangdal S, et al. Functional outcome of ankle fracture patients treated with biodegradable

implants. Foot Ankle Surg 2012;18:153-6. 28. Rikli D, et al. The potential of bioresorbable plates and screws in distal radius fracture fixation.

Injury 2002;33:S77-83. 29. Rodeo SA, et al. Tendon-healing in a bone tunnel. A biomechanical and histological study in the

dog. J Bone Joint Surg Am 1993;75:1795-803. 30. Short WH, et al. Treatment of scapholunate dissociation with a bioresorbable polymer plate: A

biomechanical study. J Hand Surg 2008;33:643-9. 31. Sinisaari IP, et al. Ruptured tibio-fibular syndesmosis: comparison study of metallic to

bioabsorbable fixation. Foot Ankle Int 2002;23:744-8. 32. Thordarson DB, et al. Bioabsorbable versus stainless steel screw fixation of the syndesmosis in

pronation-lateral rotation ankle fractures: a prospective randomized trial. Foot Ankle Int 2001;22:335-8.

33. Väänänen P, et al. The use of a biodegradable mesh plate to augment grafting of an acetabular defect - Laboratory investigation and clinical pilot study. J Bone Joint Surg Br 2010;92:179-85.

34. Väänänen P, et al. Fixation properties of a biodegradable “free-form” osteosynthesis plate with screws with cut-off screw heads: biomechanical evaluation over 26 weeks. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 2009;107:462-8.

35. Väänänen P, et al. Biomechanical in vitro evaluation of the effect of cyclic loading on the postoperative fixation stability and degradation of a biodegradable ankle plate. J Orthop Res 2008a;26:1485-8.

36. Väänänen P, et al. Fixation properties of a biodegradable "free-form" osteosynthesis plate. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 2008b;106:477-82.

http://www.ncbi.nlm.nih.gov/pubmed?term=%22Mahajan%20V%22%5BAuthor%5D

http://www.ncbi.nlm.nih.gov/pubmed?term=Volar%20fixation%20for%20dorsally%20displaced%20fractures%20of%20the%20distal%20rad

http://www.ncbi.nlm.nih.gov/pubmed?term=%22Rodeo%20SA%22%5BAuthor%5D

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