Evaluation of Different Experience Levels of Orthopaedic Residents Effect on Polymethylmethacrylate (PMMA) Bone Cement Mechanical Properties

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 Simplex™ P bone cement (Stryker Howmedica Osteonics, Mahwah, NJ) under vacuum mixing in a cement-delivery system. Tensile testing of the specimens was performed in an MTS Bionix servohydraulic 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 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 surgery, our results indicate that no special training appears to be necessary for orthopaedic residents. Rather, a basic training video demonstrating manufacturer standard procedure is all that is necessary.

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.

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 understood^1-4. 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.

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 Simplex™ 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^® cement-delivery system under a vacuum of 508 - 559 mmHg at standard operating room temperature (18 to 19°C). 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.

Figure 1. Tensile Specimen Dimensions (dimensions in mm).

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 °C).

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 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.

Figure 2. Compression Testing Apparatus.

Figure 3. Dimension drawing of the compression testing specimens.

Figure 4. Experimental Setup for Compression Testing.

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 continuously 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 (21°C).

Statistical analysis

Data retrieved for UTS and UCS of PGY-1 and PGY-5 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<0.05. Means and standard deviations were also calculated for each orthopaedic resident group. These analyses were used to determine if there were any statistical differences in mechanical properties of the bone cement between the two experience levels.

Results

Part I: Tensile Strength Testing

Figures 5 and 6 show UTS and tensile elastic modulus of the PMMA bone cement for the two groups of residents. The average UTS of the bone cement for the PGY-1 and PGY-5 residents was 38 ± 5 MPa and 39 ± 5 MPa, respectively, and there was no statistically significant difference between the two groups. When compared to data in the literature reported by Davies et al⁶, in which the UTS of the bone cement was reported to be 36 ± 10 MPa, there was no significant difference.

Figure 5. Ultimate tensile strength (UTS) from all PMMA specimens used in the study.

Figure 6. Tensile elastic modulus from all PMMA specimens used in the study.

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 there was no statistically significant difference between these two groups. When compared to the data from Davies study⁶, in which the elastic modulus of the bone cement was 2.53 ± 0.33 GPa, no significant difference was detected.

Part II: Compression Strength Testing

Figures 7 and 8 show the UCS and compression elastic modulus of the PMMA bone cement for the two groups of residents. The UCS of the bone cement for the PGY-1 and PGY-5 residents was 87 ± 6 MPa and 91 ± 5 MPa, respectively. Even though statistically there was a statistically significant difference detected between these two groups, but the difference was so small and would suggest there will be no difference in clinically. 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, and there was no statistically significant difference between these two groups.

Figure 7. Ultimate compressive strength (UCS) from all PMMA specimens used in the study.

Figure 8. Compression elastic modulus from all PMMA specimens used in the study.

Discussion

Our results suggest that no specific training during residency is needed for mixing and injecting bone cement when using Simplex™ P bone cements with the ACM cement-delivery system. This was in contradiction to our hypothesis that training during the course of residency teaches surgeons the skill of appropriately mixing and applying cement. Rather, after a brief movie and handout demonstrating the manufacturer's standard technique, interns having recently completed medical school performed as well as senior residents with experience mixing cement. For tensile strength tests, both groups of residents met the standard established by Kurtz et al⁷, who used ASTM F2118-01a with 65 mm long tensile specimens and found that the tensile elastic modulus and UTS to be 2.44 ± 0.19 GPa and 32 ± 1 MPa for Simplex™ P bone cement. There was no significant difference between the residents' tensile strength specimens or between the resident's specimens and data from Davies study⁶. For compression strength testing, there was a small but significant difference with senior residents having a higher average UCS, but the compression elastic modulus showed no difference. The resident UCS (PGY-1: 87 ± 6 MPa, and PGY-5: 91 ± 5 MPa) approached the standard from Kurtz study⁷, which used ASTM F451-99a with cylindrical compression specimens and found the UCS to be 97 ± 4 MPa for Simplex™ P bone cement. Kurtz's study used solid cylindrical compression specimens with 6 mm in diameter and 12 mm in height, whereas our study used hollow cylindrical specimens in order to mimic cement used with a prosthetic implant. From our results, we concluded that not only are both groups essentially equally skilled in cement mixing and injecting, but they are also competent, approaching or meeting Simplex™ P bone cement standards. Knowing this, cement mixing should not require additional skills lab or skills testing in the orthopaedic residency curriculum. This is in contrast to other skills that require practice and instruction for competence, such as arthroscopic technique⁵.

Our experimental design had certain limitations. First, we were limited by class size to a small number of residents and specimens tested. This could have introduced sampling bias and thus decreases the study's generalizability. Second, we limited specimen testing to compression and tensile testing. Many studies focus on fatigue failure of cement with cyclic loading as a better marker of longevity^2,4,8,9. This might be more clinically relevant to the way total knee or hip prosthesis fail when failure is related directly to the performance of the cement. Next, the metal central cores used in this study are a smooth and perfect round surface and do not represent many of the current prosthesis designs. Finally, it was assumed that senior residents had more experience with mixing and injecting cement than interns. This may not be true for the mixing cement experience as physician assistants or other operating room staffs typically mix the bone cement in our community, and therefore seniors may not have had as much more bone cement mixing experience as we initially thought. However, the senior residents do have more experience with injecting cement than interns as they have had intraoperative experience during their residency training.

To our knowledge the technical ability of residents to mix cement has not be published in the literature. Skills like these may become more important to residency programs that are trying to train competent surgeons in an environment full of restriction and standardization. Studies related to how to train and evaluate residents, are likely to follow. Additional research into cement mixing could investigate physician assistant and scrub technician competence in mixing cement as a way to ensure high quality mixing from a group with difference educational backgrounds. Studies could also see if there is a difference in more clinically relevant scenarios, such as cadaveric bone or fatigue testing.

Conflict of Interest Statement

This study did not receive any funding support for this research. However, Stryker (Kalamazoo, MI) provided the Simplex™ P bone cements and ACM cement-delivery systems used in this study, but had no role in the collection, analysis or interpretation of data, in the writing of the manuscript, or in the decision to submit the manuscript for publication. The participants and authors of this study did not receive any payments or other personal benefit, or commitments or agreements that were related in any way to the subject of the research that was conducted.