- Email: [email protected]
Mechanical testing of different hip protectors according to a European Standard
Injury, Int. J. Care Injured 40 (2009) 1172–1175
Contents lists available at ScienceDirect
Injury journal homepage: www.elsevier.com/locate/injury
Mechanical testing of different hip protectors according to a European Standard Lukas A. Holzer a,b, Gobert von Skrbensky a, Gerold Holzer a,* a b
Department of Orthopaedics, Medical University of Vienna, Waehringer Guertel 18 – 20, 1090 Vienna, Austria Center for the Advancement of Science and Art, Klagenfurt, Austria
A R T I C L E I N F O
A B S T R A C T
Article history: Accepted 9 February 2009
The effect of hip protectors depends on two factors: (1) mechanical features and (2) wearing time, which depends on the user’s compliance. Various biomechanical studies have been performed to test hip protectors’ ability in reducing impacts. However, none of these has shown the ability to simulate a fall in a natural way. Therefore, we tested conventional and newly designed hip protectors that might raise user’s compliance using a standardised mechanical test. Materials and methods: Mechanical testing was performed according to a European Standard testing method for protective impact clothing (EN 1621-1) where an impact is released vertically onto the hip protectors with an energy of 50 J. Seven different hip protectors were tested three times each. Results: Using this test, four of the hip protectors were able to reduce the impact below a suggested fracture threshold. The two newly designed hip protectors using special absorption materials were superior (9.10 kN, 12.65 kN) to the other commercially available hip protectors (21.97–50.62 kN), which differ in their mechanical testing performance. Discussion: This is the first study using a standardised mechanical test on hip protectors and allows an objective comparison because only mechanical properties are tested. New hip protectors with improved mechanical properties are superior to conventional hip protectors. Furthermore, they allow a more appealing design that increases the comfort of the wearer. Mechanical testing should be performed as a first step and has to be followed up by clinical trials to determine and clarify their overall effect. ß 2009 Elsevier Ltd. All rights reserved.
Keywords: Hip protectors Hip fractures Biomechanics Compliance Design
Hip fractures are a serious problem worldwide and their incidence is expected to rise almost threefold by the year 2050.20 The main reasons for hip fractures are a combination of osteoporosis and a fall with an impact in the area of the greater trochanter.7,15 This observation led to the introduction of hip protectors for the prevention of hip fractures that are mainly caused by sideways falls. A hip protector is a specialised piece of clothing containing pads at the side of the hip, designed to protect against impacts resulting from sideways falls.7 Basically two different types of hip protectors are available: (1) Hard hip protectors of the ‘crash-helmet’ type that shunt impact forces into the surrounding tissues and (2) soft hip protectors of the ‘energy-absorbing’ type that have the ability to absorb impacts.8 Several mechanical studies confirmed their effectiveness12,23,26,27,33,34; and the first clinical results were promising.13,19 Hip protectors were seen as a breakthrough in local-fracture prevention, 28 but recently, this paradigm changed. The effects of hip protectors depend on two factors: (1) mechanical features and (2) wearing time which depends on the
* Corresponding author. Tel.: +43 1 40400 4083. E-mail address: [email protected] (G. Holzer). 0020–1383/$ – see front matter ß 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.injury.2009.02.005
user’s compliance and adherence.3,7 One of the main problems with hip protectors is poor compliance.3,24,25 It was shown that many factors affect compliance, for example, the hip protectors’ design or wearing comfort.3,24 Therefore, recent clinical studies showed controversial results.1,2,6,18,21,24,25,29,32 The factors which are primarily involved in the effectiveness of hip protectors are their mechanical properties, which are determined by the protectors itself. In the past, however, the development of hip protectors had been completely unregulated,11,14,17,24 as a resulting, biomechanical studies showed that some hip protectors are even inefficient in in vitro conditions12,33 and other studies showed that, due to bad design, hip protectors are wrongly placed and therefore ineffective in clinical practice.4,22 Furthermore, it was demonstrated that hard protectors are more efficient in reducing impacts than soft ones33; but patients prefer soft hip protectors.10 This suggests that currently available hip protectors are not up to the patients’ needs. Therefore, Kannus et al. requested the establishment of evidence on hip protectors to handle the present situation.14 Such an evidence-based system would include the study of biomechanical characteristics of hip protectors, patients’ compliance and finally clinical trials.14 The introduction of attractive product
L.A. Holzer et al. / Injury, Int. J. Care Injured 40 (2009) 1172–1175
designs in order to increase patients’ acceptance and usage might also be a help.9,16 This study aims to test mechanically different hip protectors, for the first time, according to a European Standard for impact protectors (EN 1621-1). This method allows an objective comparison of hip protectors’ mechanical properties and furthermore will define a protective zone within the fracture threshold. Such a testing method could serve as an international standard and as the basis for an evidence-based hip protector. We tested both hard and soft hip protectors using new materials and designs. The second question in this study was whether such patientfriendly hip protectors (lower height, softer and more flexible materials) are still protective and effective; this was seen as a major concern by Kannus et al. regarding patient-friendly hip protectors.14 Materials and methods Hip protectors In this study seven different hip protectors were mechanically tested; six of these are commercially available and one is still in the concept stage, and is not yet on the market [Table 1]. The ratio of hard to soft protectors was 3:4. The hip protectors (Hips, KPH, Safehip, Safety Pants) were selected primarily due to their highest citation (for use in clinical studies) in Medline. We performed a literature search in PubMed (keywords: ‘hip protector’, ‘hip protectors’, ‘hip prot*’ and ‘protection pad’). Others (AHF, AHIP Protector) are the ones most sold in Austria according to several healthcare suppliers or orthopaedic shops in and near Vienna, Lower Austria, and Burgenland (these provinces represent about two-thirds of Austria’s population), through which we also got to test the concept hip protector (Astrosorb). The AHIP Protector1,7 KPH1,12 Safehip17 and Safety Pants112 have been described in detail elsewhere. AHF Pant1 The AHF hip protector is made of viscoelastic PUR foam. It has a roundish shape, but is narrower on the sides. The pad is constructed using two layers of foam in different shores that are connected to each other.
1173
Advanced Elastomerproducts GmbH, Austria). It has an ellipsoidal shape and a wave-like structured surface, which makes it very flexible. Furthermore, there are several air holes in the lower waves of the surface. The underpart has many spikes with flat tops. These reduce skin contact and improve ventilation. Hips1 The Hips protector’s pad consists of two shields, an outer and an inner one. The hard outer shield is designed to shunt an impact into the surrounding tissue, while the inner, more flexible one should keep the hard one in place. It has many ventilation holes and a soft rim around the outer shield to increase patient comfort. Testing apparatus We used a standardised testing method according to EN 16211:1997, the first part of a European Standard for motorcyclists’ protection clothing. This test is standardised by the European Committee for Standardization, whose members are national Norm Institutes from various European countries. This first part contains requirements for impact protectors.5 The EN 1621-1 is not used to test hip protectors in a simulated physiological falling condition. It only serves as a material testing to study and compare material properties of different hip protectors. The testing apparatus consists of two parts: (1) an impacter, representing a mass of 5000 10 g, which is made of polished steel, measuring 40 mm 80 mm and with edges having a radius of 5 mm and (2) an anvil that is connected to a sensitive piezoelectric force measuring unit (sensor: Burster series 8524; A/D converter: Dewetron 16-Kanal) with a mass of at least 1000 kg. The anvil has a hemisphere-shaped top with a radius of 50 mm. The anvil is made of polished steel and its height is 180 20 mm. The measurements are analysed using the Dasylab Version 3.5 (measX GmbH & Co. KG, Germany). When testing, the mass drops from a height of 1 m and produces a kinetic energy of 50 J at impact. Each hip protector was placed with its centre on the anvil and was tested three times. In contrast to other mechanical studies, we did not use any additional soft tissue. An effective hip protector should not exceed the suggested mean fracture threshold of 35 kN in the whole series (mean of three falls) and in a single fall/test, 50 kN.5 For this study where the primary focus is on material testing, the fracture threshold serves as a guide.
Astrosorb1 The Astrosorb hip protector is a concept-level soft protector made of a flexible patented elastomere (Astrosorb1, Astrotech
Statistical method A mean was calculated from the three falls that were simulated on each hip protector, using the peak forces of three
Table 1 Hip protectors, measurements, material and mechanism. Hip protector
Company
Size Max L/B/H in cm
Country
Material of the protector
Suggested mechanism
Hard hip protectors Hips
Qvortrup Medical A/S
16/11/2,8
Denmark
Energy-shunting/energy-absorbing
KPH
Respecta Oy
19/8,5/3,5
Finland
Safehip
Tytex Group
15,4/11/2,5
Denmark
Polycarbonate (outer) Polyethylene (inner) Outer shield of semiflexible high density polyethylene; inner shell of Plastazote Polypropylene foam; inner core with higher density
Theo Frey AG Astrotech Advanced Elastomerproducts GmbH Astrotech Advanced Elastomerproducts GmbH Raunomo Oy
18/14/2,5 16/8/2
Switzerland Austria
PUR foam Memory1 foam
Energy-absorbing Energy-absorbing
17/9/1,2
Austria
Astrosorb1
Energy-absorbing
18/16/2
Finland
Closed-cell polyethylene foam
Energy-absorbing
Soft hip protectors AHF AHIP protector Astrosorb Safety Pants
Energy-shunting/energy-absorbing
Energy-shunting/energy-absorbing
1174
L.A. Holzer et al. / Injury, Int. J. Care Injured 40 (2009) 1172–1175
Fig. 1. Hip protectors: (1) AHF Pants; (2) AHIP Protector; (3) Astrosorb; (4) Hips; (5) KPH; (6) Safehip; (7) Safety Pants.
tests STD. We used SPSS for these analyses (version 13.0; SSPS Inc, Chicago, IL). Results The results, in terms of kN peak force, are shown in Fig. 1. The horizontal line marks the fracture threshold. Below this line is the so-called ‘protective area’. For efficiency, a hip protector’s result must be below this line. Four of the seven hip protectors tested could reduce the impact below the fracture threshold. The results of the mechanical testing showed that two of the soft hip protectors (using new materials) were superior (AHIP 9.10 kN, Astrosorb 12.65 kN) to the other commercially available hip protectors (21.97–50.62 kN), which differed in performance in mechanical testing. Furthermore, the soft hip protectors were superior to the hard ones. Three of the effective protectors were made from soft material and one from hard material. The ones that failed the test were two hard ones and a soft one. Discussion Hip protectors were introduced to protect against hip fractures caused by a sideways fall. Several hard or soft hip protectors are available on the market. The results of clinical studies are controversial, especially due to low compliance. This indicates that currently available hip protectors do not fulfil the patients’ needs. Hard hip protectors showed better biomechanical properties in previous biomechanical studies compared to soft hip protectors,12,27,33 but patients prefer soft hip protectors.10 A standardised mechanical test is used for protective clothing. This study aimed to use this test for the mechanical comparison of different hip protectors. Of those tested, six are commercially available and one is still in the concept stage of trying to handle low compliance with innovative materials and patient-friendly design. The results of this study show that not all hip protectors are effective at reducing impact below a suggested fracture threshold of 35 kN. The ratio of soft to hard hip protectors tested was 4:3. Four passed the test (AHIP Protector, Astrosorb, Safehip and Safetypants); however, three failed (AHF, KPH and HIPS). Three soft protectors (AHIP Protector, Astrosorb and Safetypants) and one hard protector (Safehip) passed the test. Thus, soft protectors were superior to hard ones, in contrast to previous mechanical studies.12,27,33 Human anatomy varies between individuals. A realistic biomechanical test, therefore, should be tested on an average
human simulation or to reach higher objectivity, should be completely independent of human anatomy. Due to an unregulated development of hip protectors so far, Kannus et al. suggest that biomechanical tests should form the first step of an evidence-based hip protector model.14 Therefore, we searched for an optimal testing method, which should be a simple and internationally standardised testing method. Furthermore, it should be easily available and the testing should be cheap. Previous studies tried to simulate falls in humans,12,27,30,31,33,34 but none of these previous mechanical studies can be considered physiological.33 Test models ranged from pendulums12,27 and surrogate femurs31,33 to finite element models.30 Another important factor regarding biomechanical tests is the difference in the mechanisms of hard and soft protection pads. Hard hip protectors’ effect is partly dependent on the iliac crest onto which a major part of the energy is shunted.33 Therefore, hard hip protectors might be inadequate in situations where people are tall or where protectors are not positioned correctly.33 Positioning of hip protectors is an important factor in fracture prevention because it was shown that (especially hard) hip protectors are not correctly positioned in clinical practice and therefore are ineffective.4,22 Considering the above-mentioned issue, we suggest that, as a first step, mechanical testing of the materials should precede biomechanical studies. A standardised test should handle a worstcase scenario. This can best be assessed only by testing the hip protector or its material and its ability to reduce impacts, independently of positioning and soft tissue. We used a method that is already standardised and has been specifically developed for impact protectors, which either absorb or distribute energy or impact, on several body parts, for example, the hip. Other studies used masses varying from 5 kg30 to 36.3 kg (effective mass 40.3 kg).12 The EN 1621-1 uses a mass of 5 kg as an impacter. By adjusting the height, similar impacts can be obtained with various masses (mass velocity).12 In this study, the impacter (5 kg) drops vertically from a height of 1 m, as compared to 25 kg from a height of 8 cm (effective impact 3998 N/6378 N without soft tissue).12 The energy however is different (0.5 mass velocity2). In our test, energy of 50 J was released onto the hip protector without any additional soft tissue and therefore the impact was equal to the above-mentioned effective impact. Impacts in different studies ranged from 4330 N (with soft tissue)/3740 N (without)12 to 10 840 N (with soft tissue)/9190 N (without).12 Seven different hip protectors were mechanically tested, all of which varied in material, design or shape. According to the EN 1621-1, four hip protectors passed the test. Two protectors based on elastomeres that were specially developed for force absorption had the best results. These protectors are highly flexible and their maximum height ranges from 12 to 20 mm (up to 35 mm in others), which is seen as relatively low compared to the other tested protectors. These protectors, however, have not yet been clinically studied, but seem to be more advantageous as their mechanical properties are superior. The major advantage and strength of this study is definitely the testing method used. This test method has been standardised by the European Committee for Standardization.5 Furthermore, it is available throughout Europe, has a simple mechanism and therefore is easy to handle. The other interesting point of this study is the selection of the hip protectors tested. From the results of this study we can say that absorption should be preferred to energy shunting. It is much better to reduce the impact force than shunt it somewhere else, as we do not know (especially in cases of wrong positioning) where it is directed. This will not only have an effect on users’ compliance and adherence,
L.A. Holzer et al. / Injury, Int. J. Care Injured 40 (2009) 1172–1175
but also be more appropriate from the mechanical point of view. Soft protectors using the absorption mechanism behave independently of the anatomical structures in reducing an impact. This means that they make use of the whole pad area, in contrast to hard ones that mainly have a small protective centre, which allows maximum impact reduction. Soft hip protectors also allow a gender-neutral protection-pad design. This points to the fact that there is a need to develop new hip protectors with both adequate biomechanical properties and an attractive design in order to increase patients’ acceptance and compliance. As hip protectors are only effective when they are worn, they must not be a bulky armour. Wearing comfort and attractiveness must not suffer due to mechanical properties. A high compliance and wearing time can be obtained by finding the optimal balance between mechanical properties and attractive patient-friendly design. Conclusions We believe that testing the mechanical properties of hip protectors should be a first step in selecting effective hip protectors. This is possible using a standardised testing system. The superiority of soft hip protectors over hard ones has been shown, which can be seen as the direction for future fracture prevention. Soft hip protectors are preferred by patients and therefore increase compliance resulting in greater wearing time in clinical practice. Our results seem to be promising for the future use of hip protectors. The clinical effectiveness of newly designed hip protectors however has to be studied further. Conflict of interest statement None of the authors declares any conflict of interest. Acknowledgements Lukas A. Holzer and Gerold Holzer wish to dedicate this paper to the memory of Alfred Feichtinger (26.11.1925–26.6.2008). This study was supported in part by a research grant from the Medical University of Vienna (Fo¨rderungsstipendium der Medizinischen Universita¨t Wien, 2007) to Lukas A. Holzer. References 1. Birks YF, Porthouse J, Addie C, et al. Randomized controlled trial of hip protectors among women living in the community. Osteoporos Int 2004;15:701–6. 2. Cameron ID, Cumming RG, Kurrle SE, et al. A randomised trial of hip protector use by frail older women living in their own homes. Inj Prev 2003;9:138–41. 3. Cryer C, Knox A, Stevenson E. Factors associated with the initial acceptance of hip protectors amongst older people in residential care. Age Ageing 2006;35:72–5. 4. Derler S, Spierings AB, Schmitt KU. Anatomical hip model for the mechanical testing of hip protectors. Med Eng Phys 2005;27:475–85. 5. European Committee for Standardization. EN 1621-1:1997 Motorcyclists’ protective clothing against mechanical impact. Part 1: requirements and test methods for impact protectors.
1175
6. Heikinheimo R, Jalonen-Mannikko A, Asumaniemi H, Lehtomaki E. External hip protectors in home-dwelling older persons. Aging Clin Exp Res 2004;16:41–3. 7. Holzer G, Holzer LA. Hip protectors and prevention of hip fractures in older persons. Geriatrics 2007;62:15–22. 8. Holzer LA, Holzer G. The role of hip protectors in the prevention of hip fractures in older people. Wien Med Wochenschr 2007;157:381–7. German. 9. Holzer LA, Holzer G. Design: a neglected factor in medicine. McGill J Med 2007;10:3. 10. Honkanen LA, Dehner ML, Lachs MS. Design features to enhance external hip protector adherence in the nursing home setting. J Am Med Dir Assoc 2006;7:550–5. 11. Howland J, Peterson E, Kivell E. Hip protectors efficacy and barriers to adoption to prevent fall-related injuries in older adults: findings and recommendations from an international workgroup. J Safety Res 2006;37:421–4. 12. Kannus P, Parkkari J, Poutala J. Comparison of force attenuation properties of four different hip protectors under simulated falling conditions in the elderly: an in vitro biomechanical study. Bone 1999;25:229–35. 13. Kannus P, Parkkari J, Niemi S, et al. Prevention of hip fracture in elderly people with use of a hip protector. N Engl J Med 2000;343:1506–13. 14. Kannus P, Parkkari J, Khan K. Hip protectors need an evidence base. Lancet 2003;362:1168–9. 15. Kannus P, Sievanen H, Palvanen M, et al. Prevention of falls and consequent injuries in elderly people. Lancet 2005;366:1885–93. 16. Kannus P, Parkkari J. Prevention of hip fracture with hip protectors. Age Ageing 2006;35(Suppl. 2):ii51–4. 17. Kannus P, Parkkari J. Hip protectors for preventing hip fracture. JAMA 2007;298:454–5. 18. Kiel DP, Magaziner J, Zimmerman S, et al. Efficacy of a hip protector to prevent hip fracture in nursing home residents: the HIP PRO randomized controlled trial. JAMA 2007;298:413–22. 19. Lauritzen JB, Petersen MM, Lund B. Effect of external hip protectors on hip fractures. Lancet 1993;341:11–3. 20. Melton 3rd LJ. Epidemiology worldwide. Endocrinol Metab Clin North Am 2003;32:1–13. 21. Meyer G, Warnke A, Bender R, Muhlhauser I. Effect on hip fractures of increased use of hip protectors in nursing homes: cluster randomised controlled trial. BMJ 2003;326:76. 22. Minns RJ, Marsh AM, Chuck A, Todd J. Are hip protectors correctly positioned in use? Age Ageing 2007;36:140–4. 23. Mills NJ. The biomechanics of hip protectors. Proc Inst Mech Eng [H] 1999;210:259–66. 24. Parker MJ, Gillespie WJ, Gillespie LD. Hip protectors for preventing hip fractures in older people. Cochrane Database Syst Rev 2005;3. CD001255. 25. Parker MJ, Gillespie WJ, Gillespie LD. Effectiveness of hip protectors for preventing hip fractures in elderly people: systematic review. BMJ 2006;332: 571–4. 26. Parkkari J, Kannus P, Heikkila J, et al. Energy-shunting external hip protector attenuates the peak femoral impact force below the theoretical fracture threshold: an in vitro biomechanical study under falling conditions of the elderly. J Bone Miner Res 1995;10:1437–42. 27. Robinovitch SN, Hayes WC, McMahon TA. Energy-shunting hip padding system attenuates femoral impact force in a simulated fall. J Biomech Eng 1995;117: 409–13. 28. Rubenstein L. Hip protectors—a breakthrough in fracture prevention. N Engl J Med 2000;343:1562–3. 29. Sawka AM, Boulos P, Beattie K, et al. Hip protectors decrease hip fracture risk in elderly nursing home residents: a Bayesian meta-analysis. J Clin Epidemiol 2007;60:336–44. 30. Schmitt KU, Spierings AB, Derler S. A finite element approach and experiments to assess the effectiveness of hip protectors. Technol Health Care 2004;12:43–9. 31. Spierings AB, Derler S. Assessment of hip protectors and corresponding hip fracture risk using stress calculation in the femoral neck. Med Eng Phys 2006;28:550–9. 32. van Schoor NM, Smit JH, Twisk JW, et al. Prevention of hip fractures by external hip protectors: a randomized controlled trial. JAMA 2003;289:1957–62. 33. van Schoor NM, van der Veen AJ, Schaap LA, et al. Biomechanical comparison of hard and soft hip protectors, and the influence of soft tissue. Bone 2006;39:401–7. 34. Wiener SL, Andersson GB, Nyhus LM, Czech J. Force reduction by an external hip protector on the human hip after falls. Clin Orthop Relat Res 2002;398:157–68.
Contents lists available at ScienceDirect
Injury journal homepage: www.elsevier.com/locate/injury
Mechanical testing of different hip protectors according to a European Standard Lukas A. Holzer a,b, Gobert von Skrbensky a, Gerold Holzer a,* a b
Department of Orthopaedics, Medical University of Vienna, Waehringer Guertel 18 – 20, 1090 Vienna, Austria Center for the Advancement of Science and Art, Klagenfurt, Austria
A R T I C L E I N F O
A B S T R A C T
Article history: Accepted 9 February 2009
The effect of hip protectors depends on two factors: (1) mechanical features and (2) wearing time, which depends on the user’s compliance. Various biomechanical studies have been performed to test hip protectors’ ability in reducing impacts. However, none of these has shown the ability to simulate a fall in a natural way. Therefore, we tested conventional and newly designed hip protectors that might raise user’s compliance using a standardised mechanical test. Materials and methods: Mechanical testing was performed according to a European Standard testing method for protective impact clothing (EN 1621-1) where an impact is released vertically onto the hip protectors with an energy of 50 J. Seven different hip protectors were tested three times each. Results: Using this test, four of the hip protectors were able to reduce the impact below a suggested fracture threshold. The two newly designed hip protectors using special absorption materials were superior (9.10 kN, 12.65 kN) to the other commercially available hip protectors (21.97–50.62 kN), which differ in their mechanical testing performance. Discussion: This is the first study using a standardised mechanical test on hip protectors and allows an objective comparison because only mechanical properties are tested. New hip protectors with improved mechanical properties are superior to conventional hip protectors. Furthermore, they allow a more appealing design that increases the comfort of the wearer. Mechanical testing should be performed as a first step and has to be followed up by clinical trials to determine and clarify their overall effect. ß 2009 Elsevier Ltd. All rights reserved.
Keywords: Hip protectors Hip fractures Biomechanics Compliance Design
Hip fractures are a serious problem worldwide and their incidence is expected to rise almost threefold by the year 2050.20 The main reasons for hip fractures are a combination of osteoporosis and a fall with an impact in the area of the greater trochanter.7,15 This observation led to the introduction of hip protectors for the prevention of hip fractures that are mainly caused by sideways falls. A hip protector is a specialised piece of clothing containing pads at the side of the hip, designed to protect against impacts resulting from sideways falls.7 Basically two different types of hip protectors are available: (1) Hard hip protectors of the ‘crash-helmet’ type that shunt impact forces into the surrounding tissues and (2) soft hip protectors of the ‘energy-absorbing’ type that have the ability to absorb impacts.8 Several mechanical studies confirmed their effectiveness12,23,26,27,33,34; and the first clinical results were promising.13,19 Hip protectors were seen as a breakthrough in local-fracture prevention, 28 but recently, this paradigm changed. The effects of hip protectors depend on two factors: (1) mechanical features and (2) wearing time which depends on the
* Corresponding author. Tel.: +43 1 40400 4083. E-mail address: [email protected] (G. Holzer). 0020–1383/$ – see front matter ß 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.injury.2009.02.005
user’s compliance and adherence.3,7 One of the main problems with hip protectors is poor compliance.3,24,25 It was shown that many factors affect compliance, for example, the hip protectors’ design or wearing comfort.3,24 Therefore, recent clinical studies showed controversial results.1,2,6,18,21,24,25,29,32 The factors which are primarily involved in the effectiveness of hip protectors are their mechanical properties, which are determined by the protectors itself. In the past, however, the development of hip protectors had been completely unregulated,11,14,17,24 as a resulting, biomechanical studies showed that some hip protectors are even inefficient in in vitro conditions12,33 and other studies showed that, due to bad design, hip protectors are wrongly placed and therefore ineffective in clinical practice.4,22 Furthermore, it was demonstrated that hard protectors are more efficient in reducing impacts than soft ones33; but patients prefer soft hip protectors.10 This suggests that currently available hip protectors are not up to the patients’ needs. Therefore, Kannus et al. requested the establishment of evidence on hip protectors to handle the present situation.14 Such an evidence-based system would include the study of biomechanical characteristics of hip protectors, patients’ compliance and finally clinical trials.14 The introduction of attractive product
L.A. Holzer et al. / Injury, Int. J. Care Injured 40 (2009) 1172–1175
designs in order to increase patients’ acceptance and usage might also be a help.9,16 This study aims to test mechanically different hip protectors, for the first time, according to a European Standard for impact protectors (EN 1621-1). This method allows an objective comparison of hip protectors’ mechanical properties and furthermore will define a protective zone within the fracture threshold. Such a testing method could serve as an international standard and as the basis for an evidence-based hip protector. We tested both hard and soft hip protectors using new materials and designs. The second question in this study was whether such patientfriendly hip protectors (lower height, softer and more flexible materials) are still protective and effective; this was seen as a major concern by Kannus et al. regarding patient-friendly hip protectors.14 Materials and methods Hip protectors In this study seven different hip protectors were mechanically tested; six of these are commercially available and one is still in the concept stage, and is not yet on the market [Table 1]. The ratio of hard to soft protectors was 3:4. The hip protectors (Hips, KPH, Safehip, Safety Pants) were selected primarily due to their highest citation (for use in clinical studies) in Medline. We performed a literature search in PubMed (keywords: ‘hip protector’, ‘hip protectors’, ‘hip prot*’ and ‘protection pad’). Others (AHF, AHIP Protector) are the ones most sold in Austria according to several healthcare suppliers or orthopaedic shops in and near Vienna, Lower Austria, and Burgenland (these provinces represent about two-thirds of Austria’s population), through which we also got to test the concept hip protector (Astrosorb). The AHIP Protector1,7 KPH1,12 Safehip17 and Safety Pants112 have been described in detail elsewhere. AHF Pant1 The AHF hip protector is made of viscoelastic PUR foam. It has a roundish shape, but is narrower on the sides. The pad is constructed using two layers of foam in different shores that are connected to each other.
1173
Advanced Elastomerproducts GmbH, Austria). It has an ellipsoidal shape and a wave-like structured surface, which makes it very flexible. Furthermore, there are several air holes in the lower waves of the surface. The underpart has many spikes with flat tops. These reduce skin contact and improve ventilation. Hips1 The Hips protector’s pad consists of two shields, an outer and an inner one. The hard outer shield is designed to shunt an impact into the surrounding tissue, while the inner, more flexible one should keep the hard one in place. It has many ventilation holes and a soft rim around the outer shield to increase patient comfort. Testing apparatus We used a standardised testing method according to EN 16211:1997, the first part of a European Standard for motorcyclists’ protection clothing. This test is standardised by the European Committee for Standardization, whose members are national Norm Institutes from various European countries. This first part contains requirements for impact protectors.5 The EN 1621-1 is not used to test hip protectors in a simulated physiological falling condition. It only serves as a material testing to study and compare material properties of different hip protectors. The testing apparatus consists of two parts: (1) an impacter, representing a mass of 5000 10 g, which is made of polished steel, measuring 40 mm 80 mm and with edges having a radius of 5 mm and (2) an anvil that is connected to a sensitive piezoelectric force measuring unit (sensor: Burster series 8524; A/D converter: Dewetron 16-Kanal) with a mass of at least 1000 kg. The anvil has a hemisphere-shaped top with a radius of 50 mm. The anvil is made of polished steel and its height is 180 20 mm. The measurements are analysed using the Dasylab Version 3.5 (measX GmbH & Co. KG, Germany). When testing, the mass drops from a height of 1 m and produces a kinetic energy of 50 J at impact. Each hip protector was placed with its centre on the anvil and was tested three times. In contrast to other mechanical studies, we did not use any additional soft tissue. An effective hip protector should not exceed the suggested mean fracture threshold of 35 kN in the whole series (mean of three falls) and in a single fall/test, 50 kN.5 For this study where the primary focus is on material testing, the fracture threshold serves as a guide.
Astrosorb1 The Astrosorb hip protector is a concept-level soft protector made of a flexible patented elastomere (Astrosorb1, Astrotech
Statistical method A mean was calculated from the three falls that were simulated on each hip protector, using the peak forces of three
Table 1 Hip protectors, measurements, material and mechanism. Hip protector
Company
Size Max L/B/H in cm
Country
Material of the protector
Suggested mechanism
Hard hip protectors Hips
Qvortrup Medical A/S
16/11/2,8
Denmark
Energy-shunting/energy-absorbing
KPH
Respecta Oy
19/8,5/3,5
Finland
Safehip
Tytex Group
15,4/11/2,5
Denmark
Polycarbonate (outer) Polyethylene (inner) Outer shield of semiflexible high density polyethylene; inner shell of Plastazote Polypropylene foam; inner core with higher density
Theo Frey AG Astrotech Advanced Elastomerproducts GmbH Astrotech Advanced Elastomerproducts GmbH Raunomo Oy
18/14/2,5 16/8/2
Switzerland Austria
PUR foam Memory1 foam
Energy-absorbing Energy-absorbing
17/9/1,2
Austria
Astrosorb1
Energy-absorbing
18/16/2
Finland
Closed-cell polyethylene foam
Energy-absorbing
Soft hip protectors AHF AHIP protector Astrosorb Safety Pants
Energy-shunting/energy-absorbing
Energy-shunting/energy-absorbing
1174
L.A. Holzer et al. / Injury, Int. J. Care Injured 40 (2009) 1172–1175
Fig. 1. Hip protectors: (1) AHF Pants; (2) AHIP Protector; (3) Astrosorb; (4) Hips; (5) KPH; (6) Safehip; (7) Safety Pants.
tests STD. We used SPSS for these analyses (version 13.0; SSPS Inc, Chicago, IL). Results The results, in terms of kN peak force, are shown in Fig. 1. The horizontal line marks the fracture threshold. Below this line is the so-called ‘protective area’. For efficiency, a hip protector’s result must be below this line. Four of the seven hip protectors tested could reduce the impact below the fracture threshold. The results of the mechanical testing showed that two of the soft hip protectors (using new materials) were superior (AHIP 9.10 kN, Astrosorb 12.65 kN) to the other commercially available hip protectors (21.97–50.62 kN), which differed in performance in mechanical testing. Furthermore, the soft hip protectors were superior to the hard ones. Three of the effective protectors were made from soft material and one from hard material. The ones that failed the test were two hard ones and a soft one. Discussion Hip protectors were introduced to protect against hip fractures caused by a sideways fall. Several hard or soft hip protectors are available on the market. The results of clinical studies are controversial, especially due to low compliance. This indicates that currently available hip protectors do not fulfil the patients’ needs. Hard hip protectors showed better biomechanical properties in previous biomechanical studies compared to soft hip protectors,12,27,33 but patients prefer soft hip protectors.10 A standardised mechanical test is used for protective clothing. This study aimed to use this test for the mechanical comparison of different hip protectors. Of those tested, six are commercially available and one is still in the concept stage of trying to handle low compliance with innovative materials and patient-friendly design. The results of this study show that not all hip protectors are effective at reducing impact below a suggested fracture threshold of 35 kN. The ratio of soft to hard hip protectors tested was 4:3. Four passed the test (AHIP Protector, Astrosorb, Safehip and Safetypants); however, three failed (AHF, KPH and HIPS). Three soft protectors (AHIP Protector, Astrosorb and Safetypants) and one hard protector (Safehip) passed the test. Thus, soft protectors were superior to hard ones, in contrast to previous mechanical studies.12,27,33 Human anatomy varies between individuals. A realistic biomechanical test, therefore, should be tested on an average
human simulation or to reach higher objectivity, should be completely independent of human anatomy. Due to an unregulated development of hip protectors so far, Kannus et al. suggest that biomechanical tests should form the first step of an evidence-based hip protector model.14 Therefore, we searched for an optimal testing method, which should be a simple and internationally standardised testing method. Furthermore, it should be easily available and the testing should be cheap. Previous studies tried to simulate falls in humans,12,27,30,31,33,34 but none of these previous mechanical studies can be considered physiological.33 Test models ranged from pendulums12,27 and surrogate femurs31,33 to finite element models.30 Another important factor regarding biomechanical tests is the difference in the mechanisms of hard and soft protection pads. Hard hip protectors’ effect is partly dependent on the iliac crest onto which a major part of the energy is shunted.33 Therefore, hard hip protectors might be inadequate in situations where people are tall or where protectors are not positioned correctly.33 Positioning of hip protectors is an important factor in fracture prevention because it was shown that (especially hard) hip protectors are not correctly positioned in clinical practice and therefore are ineffective.4,22 Considering the above-mentioned issue, we suggest that, as a first step, mechanical testing of the materials should precede biomechanical studies. A standardised test should handle a worstcase scenario. This can best be assessed only by testing the hip protector or its material and its ability to reduce impacts, independently of positioning and soft tissue. We used a method that is already standardised and has been specifically developed for impact protectors, which either absorb or distribute energy or impact, on several body parts, for example, the hip. Other studies used masses varying from 5 kg30 to 36.3 kg (effective mass 40.3 kg).12 The EN 1621-1 uses a mass of 5 kg as an impacter. By adjusting the height, similar impacts can be obtained with various masses (mass velocity).12 In this study, the impacter (5 kg) drops vertically from a height of 1 m, as compared to 25 kg from a height of 8 cm (effective impact 3998 N/6378 N without soft tissue).12 The energy however is different (0.5 mass velocity2). In our test, energy of 50 J was released onto the hip protector without any additional soft tissue and therefore the impact was equal to the above-mentioned effective impact. Impacts in different studies ranged from 4330 N (with soft tissue)/3740 N (without)12 to 10 840 N (with soft tissue)/9190 N (without).12 Seven different hip protectors were mechanically tested, all of which varied in material, design or shape. According to the EN 1621-1, four hip protectors passed the test. Two protectors based on elastomeres that were specially developed for force absorption had the best results. These protectors are highly flexible and their maximum height ranges from 12 to 20 mm (up to 35 mm in others), which is seen as relatively low compared to the other tested protectors. These protectors, however, have not yet been clinically studied, but seem to be more advantageous as their mechanical properties are superior. The major advantage and strength of this study is definitely the testing method used. This test method has been standardised by the European Committee for Standardization.5 Furthermore, it is available throughout Europe, has a simple mechanism and therefore is easy to handle. The other interesting point of this study is the selection of the hip protectors tested. From the results of this study we can say that absorption should be preferred to energy shunting. It is much better to reduce the impact force than shunt it somewhere else, as we do not know (especially in cases of wrong positioning) where it is directed. This will not only have an effect on users’ compliance and adherence,
L.A. Holzer et al. / Injury, Int. J. Care Injured 40 (2009) 1172–1175
but also be more appropriate from the mechanical point of view. Soft protectors using the absorption mechanism behave independently of the anatomical structures in reducing an impact. This means that they make use of the whole pad area, in contrast to hard ones that mainly have a small protective centre, which allows maximum impact reduction. Soft hip protectors also allow a gender-neutral protection-pad design. This points to the fact that there is a need to develop new hip protectors with both adequate biomechanical properties and an attractive design in order to increase patients’ acceptance and compliance. As hip protectors are only effective when they are worn, they must not be a bulky armour. Wearing comfort and attractiveness must not suffer due to mechanical properties. A high compliance and wearing time can be obtained by finding the optimal balance between mechanical properties and attractive patient-friendly design. Conclusions We believe that testing the mechanical properties of hip protectors should be a first step in selecting effective hip protectors. This is possible using a standardised testing system. The superiority of soft hip protectors over hard ones has been shown, which can be seen as the direction for future fracture prevention. Soft hip protectors are preferred by patients and therefore increase compliance resulting in greater wearing time in clinical practice. Our results seem to be promising for the future use of hip protectors. The clinical effectiveness of newly designed hip protectors however has to be studied further. Conflict of interest statement None of the authors declares any conflict of interest. Acknowledgements Lukas A. Holzer and Gerold Holzer wish to dedicate this paper to the memory of Alfred Feichtinger (26.11.1925–26.6.2008). This study was supported in part by a research grant from the Medical University of Vienna (Fo¨rderungsstipendium der Medizinischen Universita¨t Wien, 2007) to Lukas A. Holzer. References 1. Birks YF, Porthouse J, Addie C, et al. Randomized controlled trial of hip protectors among women living in the community. Osteoporos Int 2004;15:701–6. 2. Cameron ID, Cumming RG, Kurrle SE, et al. A randomised trial of hip protector use by frail older women living in their own homes. Inj Prev 2003;9:138–41. 3. Cryer C, Knox A, Stevenson E. Factors associated with the initial acceptance of hip protectors amongst older people in residential care. Age Ageing 2006;35:72–5. 4. Derler S, Spierings AB, Schmitt KU. Anatomical hip model for the mechanical testing of hip protectors. Med Eng Phys 2005;27:475–85. 5. European Committee for Standardization. EN 1621-1:1997 Motorcyclists’ protective clothing against mechanical impact. Part 1: requirements and test methods for impact protectors.
1175
6. Heikinheimo R, Jalonen-Mannikko A, Asumaniemi H, Lehtomaki E. External hip protectors in home-dwelling older persons. Aging Clin Exp Res 2004;16:41–3. 7. Holzer G, Holzer LA. Hip protectors and prevention of hip fractures in older persons. Geriatrics 2007;62:15–22. 8. Holzer LA, Holzer G. The role of hip protectors in the prevention of hip fractures in older people. Wien Med Wochenschr 2007;157:381–7. German. 9. Holzer LA, Holzer G. Design: a neglected factor in medicine. McGill J Med 2007;10:3. 10. Honkanen LA, Dehner ML, Lachs MS. Design features to enhance external hip protector adherence in the nursing home setting. J Am Med Dir Assoc 2006;7:550–5. 11. Howland J, Peterson E, Kivell E. Hip protectors efficacy and barriers to adoption to prevent fall-related injuries in older adults: findings and recommendations from an international workgroup. J Safety Res 2006;37:421–4. 12. Kannus P, Parkkari J, Poutala J. Comparison of force attenuation properties of four different hip protectors under simulated falling conditions in the elderly: an in vitro biomechanical study. Bone 1999;25:229–35. 13. Kannus P, Parkkari J, Niemi S, et al. Prevention of hip fracture in elderly people with use of a hip protector. N Engl J Med 2000;343:1506–13. 14. Kannus P, Parkkari J, Khan K. Hip protectors need an evidence base. Lancet 2003;362:1168–9. 15. Kannus P, Sievanen H, Palvanen M, et al. Prevention of falls and consequent injuries in elderly people. Lancet 2005;366:1885–93. 16. Kannus P, Parkkari J. Prevention of hip fracture with hip protectors. Age Ageing 2006;35(Suppl. 2):ii51–4. 17. Kannus P, Parkkari J. Hip protectors for preventing hip fracture. JAMA 2007;298:454–5. 18. Kiel DP, Magaziner J, Zimmerman S, et al. Efficacy of a hip protector to prevent hip fracture in nursing home residents: the HIP PRO randomized controlled trial. JAMA 2007;298:413–22. 19. Lauritzen JB, Petersen MM, Lund B. Effect of external hip protectors on hip fractures. Lancet 1993;341:11–3. 20. Melton 3rd LJ. Epidemiology worldwide. Endocrinol Metab Clin North Am 2003;32:1–13. 21. Meyer G, Warnke A, Bender R, Muhlhauser I. Effect on hip fractures of increased use of hip protectors in nursing homes: cluster randomised controlled trial. BMJ 2003;326:76. 22. Minns RJ, Marsh AM, Chuck A, Todd J. Are hip protectors correctly positioned in use? Age Ageing 2007;36:140–4. 23. Mills NJ. The biomechanics of hip protectors. Proc Inst Mech Eng [H] 1999;210:259–66. 24. Parker MJ, Gillespie WJ, Gillespie LD. Hip protectors for preventing hip fractures in older people. Cochrane Database Syst Rev 2005;3. CD001255. 25. Parker MJ, Gillespie WJ, Gillespie LD. Effectiveness of hip protectors for preventing hip fractures in elderly people: systematic review. BMJ 2006;332: 571–4. 26. Parkkari J, Kannus P, Heikkila J, et al. Energy-shunting external hip protector attenuates the peak femoral impact force below the theoretical fracture threshold: an in vitro biomechanical study under falling conditions of the elderly. J Bone Miner Res 1995;10:1437–42. 27. Robinovitch SN, Hayes WC, McMahon TA. Energy-shunting hip padding system attenuates femoral impact force in a simulated fall. J Biomech Eng 1995;117: 409–13. 28. Rubenstein L. Hip protectors—a breakthrough in fracture prevention. N Engl J Med 2000;343:1562–3. 29. Sawka AM, Boulos P, Beattie K, et al. Hip protectors decrease hip fracture risk in elderly nursing home residents: a Bayesian meta-analysis. J Clin Epidemiol 2007;60:336–44. 30. Schmitt KU, Spierings AB, Derler S. A finite element approach and experiments to assess the effectiveness of hip protectors. Technol Health Care 2004;12:43–9. 31. Spierings AB, Derler S. Assessment of hip protectors and corresponding hip fracture risk using stress calculation in the femoral neck. Med Eng Phys 2006;28:550–9. 32. van Schoor NM, Smit JH, Twisk JW, et al. Prevention of hip fractures by external hip protectors: a randomized controlled trial. JAMA 2003;289:1957–62. 33. van Schoor NM, van der Veen AJ, Schaap LA, et al. Biomechanical comparison of hard and soft hip protectors, and the influence of soft tissue. Bone 2006;39:401–7. 34. Wiener SL, Andersson GB, Nyhus LM, Czech J. Force reduction by an external hip protector on the human hip after falls. Clin Orthop Relat Res 2002;398:157–68.