EVALUATION OF DIFFERENT EXPERIENCE LEVELS OF ORTHOPAEDIC RESIDENTS EFFECT ON POLYMETHYLMETHACRYLATE (PMMA) BONE CEMENT MECHANICAL PROPERTIES Jonathon M. Struemph, MD1, Alexander CM. Chong, MSAE, MSME1,2, Paul H. Wooley, PhD1, 2
ABSTRACT Background: PMMA bone cement is a brittle material and the creation of defects that increase porosity during mixing or injecting is a significant factor in reducing its mechanical properties. The goal during residency training is to learn how to avoid creating increased porosity during mixing and injecting the material. The aim of this study was to evaluate and compare tensile and compression strength for PMMA cement mixed by intern orthopaedic residents (PGY-1) and senior orthopaedic residents (PGY-5). The hypothesis was that the mechanical properties of PMMA cement mixed by PGY-5 would be significantly better than PMMA cement mixed by PGY-1 residents. Methods: Four PGY-1 and four PGY-5 orthopaedic residents each prepared eight tensile specimens. The bone cement used was SimplexTM P bone cement (Str yker Howmedica Osteonics, Mahwah, NJ) under vacuum mixing in a cementdeliver y system. Tensile testing of the specimens was performed in an MTS Bionix ser vohydraulic materials testing system with loading rate of 2.54 mm/min at room temperature. The mean and standard deviation of the ultimate tensile strength (UTS) for each orthopaedic resident group was calculated. The compression specimens were cylinders formed with a central core to mimic a prosthetic implant. Ten samples from each orthopaedic resident were tested using the same MTS system under identical conditions at room temperature. The specimens were loaded from -50N to complete structural failure at the rate of Department of Orthopaedics Surgery, The University of Kansas School of Medicine - Wichita, 929 N. St. Francis, Wichita, KS 67214 2 Via Christi Health - Orthopedic Research Institute, 929 N. St. Francis, Wichita, KS 67214 Investigation performed at the Via Christi Health - Orthopaedic Research Institute at Wichita, KS, USA Corresponding Author: Alexander Chong, Research Engineer Via Christi Health - Orthopaedic Research Institute 929 N. St. Francis, Wichita, KS 67214 USA Phone: 316-268-5462 Fax: 316-291-4998 [email protected] 1
20 mm/min. The ultimate compressive strength (UCS) was then determined and the mean and standard deviation calculated for each group. Results: The average UTS of the bone cement for the PGY-1 and PGY-5 residents was 37.5 ± 4.5 MPa and 39.2 ± 5.0 MPa, respectively, and there was no statistically significant difference between the two groups. For the tensile elastic modulus of the bone cement, the results for the PGY-1 and PGY-5 residents were 2.40 ± 0.09 GPa and 2.44 ± 0.08 GPa, respectively, and again there was no statistically significant difference. For the compression elastic modulus of the bone cement, the results for the PGY-1 and PGY-5 residents were 1.19 ± 0.13 GPa and 1.21 ± 0.18 GPa, respectively, with no statistically significant difference. However, the UCS of the bone cement for the PGY-1 and PGY-5 residents was 87.4 ± 5.8 MPa and 91.1 ± 4.5 MPa, respectively, and there was a statistically significant difference between the groups. Discussion: The PMMA specimens prepared by both the PGY-1 and PGY-5 resident groups had similar characteristics during tensile and compression testing, and were similar to known standards. Although mixing and applying bone cement is an important skill for joint replacement surger y, our results indicate that no special training appears to be necessar y for orthopaedic residents. Rather, a basic training video demonstrating manufacturer standard procedure is all that is necessar y. Clinical Relevance: The results of this study indicate the importance of experience in bone cement mixing and injecting on cement mechanical properties, but indicate that no special training appears to be necessary for orthopaedic residents. Keywords: Polymethylmethacr ylate (PMMA); Bone cement; Mechanical behavior; Experiences; Orthopaedic Resident Education
Volume 35 193
J. M. Struemph, A. C. M. Chong, P. H. Wooley Figure 1. Tensile Specimen Dimensions (dimensions in mm)
Figure 1. Tensile Specimen Dimensions (dimensions in mm)
INTRODUCTION Each orthopaedic resident should become familiar with the basic technique for mixing polymethylmethacrylate (PMMA) bone cement and to understand the impact of PMMA preparation. Although orthopaedic surgeons frequently delegate the mixing of PMMA bone cement to physician assistants, nurse practitioners, or scrub nurses, the surgeon is responsible for ensuring it is performed correctly. PMMA bone cement is a brittle material and the creation of defects that increase porosity during mixing or injecting is a significant factor in reducing its mechanical properties. Porosity has been shown to negatively affect the fatigue life of bone cements. However, vacuum mixing (widely used to reduce porosity and pore size in the clinical setting) has had mixed results that suggests the effects of porosity are not completely understood1-4. Thus the technique used or the practitioner’s experience in mixing PMMA bone cement could potentially influence the clinical outcome of cemented prostheses. Orthopaedic resident education relies on directed reading, didactic sessions, skills labs, and surgical experience to train capable orthopaedic surgeons. Best practice in resident training is an understudied topic with increasing relevance in a culture demanding standardization. This is compounded by the growing need to test resident skills as work hour restrictions change the way residents are trained and evaluated5. PMMA bone cement mixing at our institution is taught primarily through intraoperative experience. It is possible that more instruction may be needed in areas such as cement mixing and cement injecting, which are incorporated differently in different institutions. Comparing experienced and inexperienced residents’ skills could identify potential shortcomings in the educational process. Our study was conducted to evaluate and compare the level of training needed for mixing and injecting bone cement by comparing more experienced senior residents with less experienced interns. The aim was to detect any differences in mechanical properties (tensile and compression strength) of the bone cement and correlate this with experience level.
194 The Iowa Orthopaedic Journal
MATERIALS AND METHODS Four intern orthopaedic residents (PGY-1) and four senior orthopaedic residents (PGY-5) were involved in this study. The PGY-1 residents have no prior training or experience for mixing PMMA bone cement, while the PGY-5 residents have previously trained in skills labs and mixed at least 50 times during the 5 years residency education. The PMMA bone cement used in this study was SimplexTM P bone cement (Stryker Howmedica Osteonics, Mahwah, NJ) under vacuum mixing in a cement-delivery system (Advanced Cement Mixing (ACM), Stryker Instruments, Kalamazoo, MI). Universal precautions were followed in accordance with Occupational Safety and Health Administration standards. Both groups of residents viewed a 3-minute training video (Stryker Howmedica Osteonics, Mahwah, NJ) for using the Stryker® ACM cement-delivery system and were provided a 5-minute reading period for the modified manufacturer standard procedure printout. Two types of mechanical properties of the bone cement were investigated for this study: 1) tensile strength and 2) compression strength. Part I: Tensile Strength Testing A custom designed tensile specimen mold from the Orthopaedic Research Institute (ORI) lab, which created four standard tensile specimens, was used. Figure 1 shows the dimensions of each tensile testing specimen, and the thickness of the samples was 3 mm (0.12 inches). Eight samples from each orthopaedic resident were tested for a total of 64 samples. The PMMA bone cement specimens were prepared by following the modified manufacturer standard procedure, which includes vacuum mixing for 90 seconds in a Stryker® cementdelivery system under a vacuum of 508 - 559 mmHg at standard operating room temperature (18 to 19oC). The cement was then transferred into the polyethylene tensile specimen molds using the cement injection gun. Care was taken not to trap air in the cement during the injection process by avoiding the layering of cement. These specimens were allowed to cure in the mold for 24 hours, and then were removed. The tested regions of all the specimens (cross-sectional area) were measured using a digital caliper, and radiographs of all the specimens were taken. Tensile testing was performed using a MTS Bionix servohydraulic materials testing system (MTS Model 858, Eden Prairie, MN). The specimens were loaded from 0 N to complete structural failure at the stroke rate of 2.54 mm/ min. Load and deflection data were recorded continuously at 10 Hz. The ultimate tensile strength (UTS) and the tensile elastic modulus (E) were then determined. The mean and standard deviation of the groups were calculated for each orthopaedic resident. This mechanical test was performed in air at room temperature (21 ºC).
Polymethylmethacrylate (PMMA) Bone Cement Mechanical Properties Figure 4. Experimental Setup for Compression Testing Figure 2. Compression Testing Apparatus
Figure 2. Compression Testing Apparatus Figure 3. Dimension drawing of the compression testing specimens
Figure 4. Experimental Setup for Compression Testing Figure 3. Dimension drawing of the compression testing specimens
Part II: Compression Strength Testing A custom designed compression testing specimen mold from the ORI lab was used (Figure 2), and Figure 3 shows the dimensions of the specimens. The specimens were made of PMMA cement surrounding a central core cylinder made from a professionally machined stainless steel rod intended to mimic a prosthetic implant. This rod was left in place for all of the samples during testing but only the cement was compressed during the testing (Figure 4). The cement was prepared by following the A manufacturer’s standard procedure, and then injected into the appropriate polyethylene molds. These specimens were allowed to cure in the mold for 24 hours, and then were removed. All compression testing was performed with strict adherence to the American Society for Testing and Materials (ASTM) F451-99a standards - Standard Specification for Acrylic Bone Cement. The dimensions for each cylindrical-shaped specimen was measured using a digital caliper and was recorded prior to testing. Ten samples from each orthopaedic resident were tested, for a total of 80 samples. All the specimens were tested in compression using the MTS Bionix servohydraulic materials testing system. The specimens were loaded from -50N to complete structural failure at the rate of 20 mm/min. Load and deflection data were recorded con-
tinuously at 10 Hz. The ultimate compressive strength (UCS) and the compression elastic modulus were then determined. The mean and standard deviation were calculated for each orthopaedic resident’s group. All mechanical tests were performed in air at room temperature (21ºC). Statistical analysis Data retrieved for UTS and UCS of PGY-1 and PGY5 residents were analyzed using one-way analysis of variance (ANOVA) of SPSS software (Version 16.0; SPSS, Chicago, IL) with the Least Significant Difference (LSD) multiple comparisons post hoc analysis. The level of significance was defined as p
ABSTRACT Background: PMMA bone cement is a brittle material and the creation of defects that increase porosity during mixing or injecting is a significant factor in reducing its mechanical properties. The goal during residency training is to learn how to avoid creating increased porosity during mixing and injecting the material. The aim of this study was to evaluate and compare tensile and compression strength for PMMA cement mixed by intern orthopaedic residents (PGY-1) and senior orthopaedic residents (PGY-5). The hypothesis was that the mechanical properties of PMMA cement mixed by PGY-5 would be significantly better than PMMA cement mixed by PGY-1 residents. Methods: Four PGY-1 and four PGY-5 orthopaedic residents each prepared eight tensile specimens. The bone cement used was SimplexTM P bone cement (Str yker Howmedica Osteonics, Mahwah, NJ) under vacuum mixing in a cementdeliver y system. Tensile testing of the specimens was performed in an MTS Bionix ser vohydraulic materials testing system with loading rate of 2.54 mm/min at room temperature. The mean and standard deviation of the ultimate tensile strength (UTS) for each orthopaedic resident group was calculated. The compression specimens were cylinders formed with a central core to mimic a prosthetic implant. Ten samples from each orthopaedic resident were tested using the same MTS system under identical conditions at room temperature. The specimens were loaded from -50N to complete structural failure at the rate of Department of Orthopaedics Surgery, The University of Kansas School of Medicine - Wichita, 929 N. St. Francis, Wichita, KS 67214 2 Via Christi Health - Orthopedic Research Institute, 929 N. St. Francis, Wichita, KS 67214 Investigation performed at the Via Christi Health - Orthopaedic Research Institute at Wichita, KS, USA Corresponding Author: Alexander Chong, Research Engineer Via Christi Health - Orthopaedic Research Institute 929 N. St. Francis, Wichita, KS 67214 USA Phone: 316-268-5462 Fax: 316-291-4998 [email protected] 1
20 mm/min. The ultimate compressive strength (UCS) was then determined and the mean and standard deviation calculated for each group. Results: The average UTS of the bone cement for the PGY-1 and PGY-5 residents was 37.5 ± 4.5 MPa and 39.2 ± 5.0 MPa, respectively, and there was no statistically significant difference between the two groups. For the tensile elastic modulus of the bone cement, the results for the PGY-1 and PGY-5 residents were 2.40 ± 0.09 GPa and 2.44 ± 0.08 GPa, respectively, and again there was no statistically significant difference. For the compression elastic modulus of the bone cement, the results for the PGY-1 and PGY-5 residents were 1.19 ± 0.13 GPa and 1.21 ± 0.18 GPa, respectively, with no statistically significant difference. However, the UCS of the bone cement for the PGY-1 and PGY-5 residents was 87.4 ± 5.8 MPa and 91.1 ± 4.5 MPa, respectively, and there was a statistically significant difference between the groups. Discussion: The PMMA specimens prepared by both the PGY-1 and PGY-5 resident groups had similar characteristics during tensile and compression testing, and were similar to known standards. Although mixing and applying bone cement is an important skill for joint replacement surger y, our results indicate that no special training appears to be necessar y for orthopaedic residents. Rather, a basic training video demonstrating manufacturer standard procedure is all that is necessar y. Clinical Relevance: The results of this study indicate the importance of experience in bone cement mixing and injecting on cement mechanical properties, but indicate that no special training appears to be necessary for orthopaedic residents. Keywords: Polymethylmethacr ylate (PMMA); Bone cement; Mechanical behavior; Experiences; Orthopaedic Resident Education
Volume 35 193
J. M. Struemph, A. C. M. Chong, P. H. Wooley Figure 1. Tensile Specimen Dimensions (dimensions in mm)
Figure 1. Tensile Specimen Dimensions (dimensions in mm)
INTRODUCTION Each orthopaedic resident should become familiar with the basic technique for mixing polymethylmethacrylate (PMMA) bone cement and to understand the impact of PMMA preparation. Although orthopaedic surgeons frequently delegate the mixing of PMMA bone cement to physician assistants, nurse practitioners, or scrub nurses, the surgeon is responsible for ensuring it is performed correctly. PMMA bone cement is a brittle material and the creation of defects that increase porosity during mixing or injecting is a significant factor in reducing its mechanical properties. Porosity has been shown to negatively affect the fatigue life of bone cements. However, vacuum mixing (widely used to reduce porosity and pore size in the clinical setting) has had mixed results that suggests the effects of porosity are not completely understood1-4. Thus the technique used or the practitioner’s experience in mixing PMMA bone cement could potentially influence the clinical outcome of cemented prostheses. Orthopaedic resident education relies on directed reading, didactic sessions, skills labs, and surgical experience to train capable orthopaedic surgeons. Best practice in resident training is an understudied topic with increasing relevance in a culture demanding standardization. This is compounded by the growing need to test resident skills as work hour restrictions change the way residents are trained and evaluated5. PMMA bone cement mixing at our institution is taught primarily through intraoperative experience. It is possible that more instruction may be needed in areas such as cement mixing and cement injecting, which are incorporated differently in different institutions. Comparing experienced and inexperienced residents’ skills could identify potential shortcomings in the educational process. Our study was conducted to evaluate and compare the level of training needed for mixing and injecting bone cement by comparing more experienced senior residents with less experienced interns. The aim was to detect any differences in mechanical properties (tensile and compression strength) of the bone cement and correlate this with experience level.
194 The Iowa Orthopaedic Journal
MATERIALS AND METHODS Four intern orthopaedic residents (PGY-1) and four senior orthopaedic residents (PGY-5) were involved in this study. The PGY-1 residents have no prior training or experience for mixing PMMA bone cement, while the PGY-5 residents have previously trained in skills labs and mixed at least 50 times during the 5 years residency education. The PMMA bone cement used in this study was SimplexTM P bone cement (Stryker Howmedica Osteonics, Mahwah, NJ) under vacuum mixing in a cement-delivery system (Advanced Cement Mixing (ACM), Stryker Instruments, Kalamazoo, MI). Universal precautions were followed in accordance with Occupational Safety and Health Administration standards. Both groups of residents viewed a 3-minute training video (Stryker Howmedica Osteonics, Mahwah, NJ) for using the Stryker® ACM cement-delivery system and were provided a 5-minute reading period for the modified manufacturer standard procedure printout. Two types of mechanical properties of the bone cement were investigated for this study: 1) tensile strength and 2) compression strength. Part I: Tensile Strength Testing A custom designed tensile specimen mold from the Orthopaedic Research Institute (ORI) lab, which created four standard tensile specimens, was used. Figure 1 shows the dimensions of each tensile testing specimen, and the thickness of the samples was 3 mm (0.12 inches). Eight samples from each orthopaedic resident were tested for a total of 64 samples. The PMMA bone cement specimens were prepared by following the modified manufacturer standard procedure, which includes vacuum mixing for 90 seconds in a Stryker® cementdelivery system under a vacuum of 508 - 559 mmHg at standard operating room temperature (18 to 19oC). The cement was then transferred into the polyethylene tensile specimen molds using the cement injection gun. Care was taken not to trap air in the cement during the injection process by avoiding the layering of cement. These specimens were allowed to cure in the mold for 24 hours, and then were removed. The tested regions of all the specimens (cross-sectional area) were measured using a digital caliper, and radiographs of all the specimens were taken. Tensile testing was performed using a MTS Bionix servohydraulic materials testing system (MTS Model 858, Eden Prairie, MN). The specimens were loaded from 0 N to complete structural failure at the stroke rate of 2.54 mm/ min. Load and deflection data were recorded continuously at 10 Hz. The ultimate tensile strength (UTS) and the tensile elastic modulus (E) were then determined. The mean and standard deviation of the groups were calculated for each orthopaedic resident. This mechanical test was performed in air at room temperature (21 ºC).
Polymethylmethacrylate (PMMA) Bone Cement Mechanical Properties Figure 4. Experimental Setup for Compression Testing Figure 2. Compression Testing Apparatus
Figure 2. Compression Testing Apparatus Figure 3. Dimension drawing of the compression testing specimens
Figure 4. Experimental Setup for Compression Testing Figure 3. Dimension drawing of the compression testing specimens
Part II: Compression Strength Testing A custom designed compression testing specimen mold from the ORI lab was used (Figure 2), and Figure 3 shows the dimensions of the specimens. The specimens were made of PMMA cement surrounding a central core cylinder made from a professionally machined stainless steel rod intended to mimic a prosthetic implant. This rod was left in place for all of the samples during testing but only the cement was compressed during the testing (Figure 4). The cement was prepared by following the A manufacturer’s standard procedure, and then injected into the appropriate polyethylene molds. These specimens were allowed to cure in the mold for 24 hours, and then were removed. All compression testing was performed with strict adherence to the American Society for Testing and Materials (ASTM) F451-99a standards - Standard Specification for Acrylic Bone Cement. The dimensions for each cylindrical-shaped specimen was measured using a digital caliper and was recorded prior to testing. Ten samples from each orthopaedic resident were tested, for a total of 80 samples. All the specimens were tested in compression using the MTS Bionix servohydraulic materials testing system. The specimens were loaded from -50N to complete structural failure at the rate of 20 mm/min. Load and deflection data were recorded con-
tinuously at 10 Hz. The ultimate compressive strength (UCS) and the compression elastic modulus were then determined. The mean and standard deviation were calculated for each orthopaedic resident’s group. All mechanical tests were performed in air at room temperature (21ºC). Statistical analysis Data retrieved for UTS and UCS of PGY-1 and PGY5 residents were analyzed using one-way analysis of variance (ANOVA) of SPSS software (Version 16.0; SPSS, Chicago, IL) with the Least Significant Difference (LSD) multiple comparisons post hoc analysis. The level of significance was defined as p
