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Stress-Strain Curve for Bones | Experiment

Introduction In human and animal body there are more than 200 bones present, including long, short, flat, sesamoid and irregular bone. The osteogenic cells, organic matrix and mineral are three major components in the bone. Osteoblasts, osteoclasts and the osteocytes cell are osteogenic cells; they involved the bone regeneration process. Collagen and proteoglycan are the main organic component in the bone matrix, they provide elasticity and flexibility of the bone. Bone mineral mainly hydroxyapatite (Ca5(PO4)3(OH)), provides the mechanical rigidity to the bone and also give some load bearing strength to the bone. In human and animal bodies has so many functions in the day to day life, therefore bone is involve the damage due to same forces. Because of that each and every bone in the body is undergoing remodelling process to adapt the biomechanical forces and removal of damaged bone. In a human child, during their childhood, his or her bone mass has to increased and it will become peak level between the age of 18-35yreas. The genetic factor mainly involved the bone mass peak level, other than that nutrition, body activities, behaviours and pharmaceutical agents also involving the bone mass peak level. After that resorption and remodelling process happening equilibrium, if there are any of the processes increased that lead to a disease condition.
The bone mass composed of 80% of cortical bone and 20% of cancellous bone. The cancellous bone (trabecular bone or spongy bone) is very light bone, consist of some spaces and give honeycombed or spongy appearance to the bone. Bone matrix is filling with those spaces and highly vascularised than the compact bone. Due to this structure cancellous bone is providing structural support and flexibility to the whole bone without the compact bone. Therefore depending on the strength and flexibility need of the body cancellous bone was arranged on the bone, but it is not tolerated too much of the stress. It is mainly found at the ends of the long bone. Tibia bone is one of the long bone on lower hind leg bones; it is the largest bone in the lower hind leg bone and it functions is load bearing in the body, this bone reluctance to the stress fracture. Therefore analysis of the stress forces on the body tibia is a good example, but can’t use the human bone for that purposes. In that situation can use animal bone models, those will provide uniformed experimental material to experiment and also allow to test some disease condition and drug therapies of a human. The scientist used verities of animal for those purposes including large animal like cow and pig, test animal like guinea pig and rabbits, and some avian. This testing experiment used bovine and porcine tibial, cancellous bone and those bones undergone some mechanical load, from that try to observe if cancellous bone strength and stiffness depending on the bone density, animal species and under testing condition.
The stress-strain curve is given to characterise the behaviour for mechanical properties of the biomaterials, according to the stress-strain curve bone is the brittle material. Therefore bone has compressive stress proportionate to strain limit, elastic limit and breaking strength, to understand those properties of the bone have to perform the experiment with the compressive or tensile load. The understanding of those limits is very essential, because it helps to develop the new biomaterials similar to the bone, and it gives advantages for future bone disease treatment. E.g. osteoporosis
ASTM D1621 and ISO 844 Standards This is the standard testing methods regarding the behaviour of the cellular materials under mechanical compressive loads. This case try to figure out the under maximum load compressive strength or limit load proportionate to compressive stress, by doing this can calculate the elasticity modulus for test material. But this methods required some specific reference value, by using those value can created the standard for materials. Therefore it can performed the compressive test for every materials produced the standard data of all materials, it is helpful for material development and research for new materials, quality control of materials, acceptance or rejection of materials by using specification of standards. There are practical protocol always check before loading to specimen;
Specimen should be parallel to each other
Specimen has to perpendicular to the sides
Specimen surfaces should be imperfection
Anisotropy direction of compressive load
Each experiment at least five specimen tested
Other than those above mention protocol there are some protocol also should check before performing the test.
Materials and methods Materials
Slices of bone samples (approximately 5mm) from bovine or porcine proximal tibias
Band saw
Core drill
Digital Vernier calipers (MW110-15DDL, Moore and Wright, UK)
Digital balance (CM 60-2N, Kern and Sohn, Germany)
Testing machine (Testing machine by ESH Testing Ltd, Brierley Hill, UK)
LVDT which permits measurements to a precision of /- 0.1%
Load cell permits measurements to a precision of /-1%
Ethanol
Methods Thin slices (approximately 5mm) of 12 bone samples each from bovine and porcine proximal tibias were collected using a band saw.
The cancellous bone of approximately 9mm diameter was obtained from these samples using a core drill
The sample dimensions were measured using digital Vernier Calipers.
Each sample is placed in a vial containing ethanol for a week to dissolve the bone marrow.
The ethanol was changed every 3-4 days.
After one week the samples were removed from the ethanol, air-dried and measured the weights using a digital balance.
The samples from each species were divided into two groups of 6 each.
One group was subjected to “unconstrained loading” and the other group to “constrained loading”.
The test specimens were conditioned at 23 /-20C and 50 /-5% relative humidity for 40hr prior to testing
The test was conducted in a standard laboratory atmosphere of 23 /-20C and 50 /-5% relative humidity.
Unconstrained loading:
Unconstrained loading test was conducted on 6 samples each from cows and pigs.
The sample was placed in a flat surface in a material testing machine and compressed using a flat ended surface of 25mm diameter at a rate of 0.5mm/min. (ASTM D 1621 and ISO 844 Standards)
The displacement of the flat- ended surface and the applied force were measured using an LVDT and load cell.
The displacement and force were digitally sampled at a frequency of 5Hz and stored on a PC.
Testing was continued until a yield point or a deformation of approximately 0.65mm was reached. (ASTM D 1621 and ISO 844 Standards)
Constrained loading:
Constrained loading was conducted on 6 samples each of bovine and porcine.
The sample was placed within an aluminum cylinder with an internal diameter of 9mm and a height of 5mm.
The cylinder with sample was placed on a flat surface in a material testing machine and was compressed using a flat-ended surface of diameter 8.9mm at a rate of 0.5mm/min. (ASTM D 1621 and ISO 844 Standards)
The displacement of the flat- ended surface and the applied force were measured using an LVDT and load cell.
The displacement and force were digitally sampled at a frequency of 5Hz and stored on a PC.
Result All the data collected arranged in the EXCEL data sheet; in there consisted of force and displacement data for all 24 samples. In that 12 sample for bovine and other 12 samples for porcine, both had constrained and unconstrained loading. The stress and strain for each sample at different times were calculated using the below formula or given EXCEL formulary.
Strain = displacement /thickness
Stress = Force/cross sectional area (Ï€r2)
The stress-strain curve graph for each sample was drawn using EXCEL formulary. The stiffness, compressive strength, zero strain point and failure strain were calculated by using those graphs. The maximum stress at the first hump was taken as the compressive strength even though the stress often rises to a larger value again at higher than the first hump as the first hump is the point where the pore structure first start to fail.
Compressive modulus or stiffness was calculated by dividing the stress and strain at any point on the straight portion of graph. The zero strain point was measured by extending a straight line from the steepest straight potion of the stress-strain curve towards the zero load line. The compressive failure strain is the strain at compressive failure minus the zero strain at which the failure occurs.
Bone density is determined by using the principle of densitometry, when the volume and bone weight are both known the bone density can calculate by using following equation for each samples.
Bone density=bone mass (Kg)/ bone volume (l)
(We measured bone mass as a gram (gr) and volume as a millimetre3 (mm3), it is very small therefore we converted in the density for gram/ centimetre3. And also mass of the bone took without bone marrow therefore this density must be apparent density for each of the sample).
By using EXCEL data we got stiffness, compressive strength, failure strain and zero strain point for all 24 test samples.
By using above data can analyzed stiffness, compressive strength and compressive failure strain of each cancellous bone depend on the density, and also got the analytical data for difference between the average stiffness, compressive strength and compressive failure strain of cancellous bone from cows and pig according to the density. Other than the above mention data can differentiate between the average failure strain of cancellous bone when tested in unconstrained and constrained mode according to the bone density.
The average stiffness, compressive strength and compressive failure strain of cancellous bone from cows and pigs were plotted against their apparent density and compared the regression lines. Similarly, the average stiffness, compressive strength and compressive failure strain of cancellous bone from cows and pigs tested in constrained and unconstrained mode were plotted against their apparent density and compared the regression lines.
Those analytical data explain in the below.
Consider stiffness of the bovine and porcine bones Under unconstrained loading, the stiffness of bovine bones shows a linear increase with increase in apparent density (slope=678.87). Where as bovine bone shown the stiffness decreases with apparent density in constrained loading (slope=-129.72). (Figure 01

Obesity: Effect on Total Joint Replacement Patients

The critical factor driving the growth in worldwide demand for joint replacement is obesity Kumar Anjan

Contents (Jump to)
Abstract:
1. Introduction
2. Obesity – How can we define it?
3. Surgical Risk:
4. Obesity and Implant Failure
5. Conclusion
6. Bibliography

Abstract: During early days, obese individuals were often suggested to lose weight before undergoing total joint replacement (TJR). It was common observation amongst surgeons and doctors that morbidity rate amongst obese individuals were significantly high as compared to that of non-obese subjects. In addition, there was significant increase in the physical and technical labour of operating overweight individuals. This resulted in time saving and managing long queue of patients. Recently, scientific reports with positive results reflected that there is only negligible effect of obesity on TJR. However, recently in the UK several health care authorities proposed that there would not be any financial support provided to the individuals whose BMI exceeds 30 kg/m2. The primary reason behind the decision is the reduction in health care budget. In olden days, TJR was a procedure considered for those who were more than 65 years of age. However, this trend is significantly changing. According to Dr. Ayeres (MD, Chair in Orthopaedics, and director of the Musculoskeletal Centre of Excellence at UMass Medical School), with an increased rate of obesity amongst individuals under the age of 65 is acting as a driving force towards TJR. Therefore, in this case report I have discussed about obesity and its effect on TJR.
1. Introduction: Total joint replacement (TJR) is globally acknowledged especially due to the revolution in the quality of life for those individuals suffering from osteoarthritis or similar health problems (Garellick et al, 1998). Moreover, in modern medicine TJR has proved its effectiveness as one of the most successful interventions. There are also several high demo graphs recorded towards the improvement of the quality of life, which surpasses coronary artery bypass as well as renal transplants (Williams A, 1985). In elderly population, TJR’s especially knee anthroplasty has shown to be most effective technology resulting towards better life quality. Study conducted among a population cohort of over 65 subjects who had TJR shows that they are leading a healthier life (UK population Census, 2001).
Total joint replacement has definitely bought a revolution in modern health care system. However, there are certain implications that concern the public. One of the most critical limitations is the budgetary control which enforced by the competitive claim from the other intensive medical care system. Furthermore, as these treatments are not actually cost effective; therefore, it raises questions for the individuals undergoing a replacement as well as the government bodies who support the funding (Templeton, S.K. 2005). Recently, East Suffolk health trust in the U.K. decided to prioritize their patients undergoing TJR according to their weight and various other factors resulting in obesity. According to the top management of the trust, individuals who are overweight or obese are at an increased risk towards the efficacy of the surgery. This decision has definitely stirred controversy among the community undergoing TJR (Finer N, 2005). However, according to some valued sources, there is no evidence that age, obesity or gender affects the functional outcome of the surgery (Templeton, S. K. 2005). Therefore, there is huge controversy surrounding towards the potential implications of obesity on TJR.
Orthopaedic studies suggest that obesity leads towards degenerative changes in joints and leads towards complications and functional risk during post-surgery phase (Rockville, 2003). As there is no standard definition for obesity, it rather becomes very difficult to understand its actual meaning. However, several health care professionals recommend that problem in mobilisation and functional outcome is not visualised until an individual’s (BMI) exceeds 40 kg/m2 (Nammi et al, 2004). Various evidences conclude that obesity is the driving force towards development of osteoarthritis particularly in individuals with high BMI in an early age (DoH, 2001). In some rare scenarios, bariatic surgery is performed on the individuals before TJR. This is mainly due to bring their weight down to an acceptable score.
2. Obesity – How can we define it? Over several years, different authors described obesity in a different way. Obesity does not have an actual standard definition. However, the most common scientific way to describe obesity is based on the Body Mass Index (BMI) (Fig: 1) (Lawrence, 1998). BMI is also known as Quetelet Mass Index (QI) and is generally described as the ratio of the square of the height measured in meters (mt) to the weight in kilograms (kg) (Taylor, 1998). QI relates the body fat percentage and is one of the most preferred methods for the assessment of the potential health risk related with the overweight or obesity. Recently, authors started using the term “New World Syndrome” for obesity as its prevalence is dramatically increasing in the Europe as well as in the United States (USA). A shocking figure was projected when a recent survey was conducted by the Department of Health in the UK. According to the survey, prevalence in obesity has increased from 15% since 1995 to 21% in 2001 (Webb et al, 2004).

Fig: 1 BMI Chart the ratio of the square of the height measured in meters (mt) to the weight in kilograms (kg).
In the US, obesity has reached in an epidemic proportion. Considering the BMI of an individual, more than half of the adult population in the US are classified as overweight. According to a separate survey conducted in the US amongst 65-74 year age group, 66% were referred to as obese or overweight. Therefore, we can visualise the prevalence of obesity coinciding with the peak age during which most of the individual requires TJR (US Dept. of Health and Human Services, 2003). In the UK, the data shows similar outcomes to that of the US. Obesity amongst males in the UK has increased from 6% in 1980 to 22% in 2002 whereas in females, 8% – 23% (DhO, 2001). According to the World Health Organisation (WHO), there is an increase in obesity between 10% – 40% in last 10 years. WHO also claims that there are approximately 200 million obese adults around the globe and 18 million children under age five are classified as overweight. Moreover, by 2000 this data significantly increased to over 300 million.
Osteoarthritis (OA) is a group of mechanical abnormalities, which involves in the degradation of joints, articular cartilage. It generally affects approximately 20 million individuals in the US. It causes substantial morbidity leading to disability in the later stages. This disease is more common amongst elderly population. However, recently it was observed that adult age group between 60-65 years of age are getting prone to this disease. According to few scientific sources, the main reason for OA amongst younger generation is obesity. Various scientific reports documents that in the US more than 200,000 knee and hip replacements are performed each year and 35% are young individuals under the age of 65 (Dho, 2001; US Dept. of Health and Human Services, 2003).
Obesity is one of the most significant risk factors contributing towards osteoarthritis. Therefore, with an increase in obesity, there is a high probability of developing osteoarthritis. Moreover, this leads towards an increase prevalence of TJR (Felson et al, 2000). As we know that, there is a constant increase among obese patients undergoing TJR. Therefore, several researches links obesity with the TJR as well as the complications associated with the same. According to a joint study performed by a group of scientists and surgeons, it was found that there is an increase in complication rate in obese patients as compared to individuals with normal BMI (Olivera et al, 1999; Sahyoun et al, 1999). In addition, the operative duration significantly increases in obese subjects. However, factors like physical stress and injury to health care professional remains undiscovered. As already mentioned, it has been well established that there is a positive link that connects TJR and obesity. Whilst examining, individuals with high BMI are in an exponential increase for TJR over next few decades. According to several health care professionals, there is often a challenging situation during pre/post surgery in obese individuals. Moreover, there is a high risk of blood loss and blood transfusion. It has also been highlighted that nerve injury is common amongst obese patients as compared to the healthy individuals (non-obese) during TJR (Mantilla et al, 2003).
3. Surgical Risk: In the previous section, it was discussed that East Suffolk Health Trust in the UK prioritised their patients, which resulted in a huge controversy. According to public and human right activists, their decision was biased towards the individuals with higher BMI. The main reason behind the decision was increased risk and the cost involved in performing TJR amongst obese/overweight individuals. Supporting the decision of East Suffolk Health Trust, “Ipswich Protocol” was followed. According to this protocol, orthopaedic surgeons and health care personals were advised that patients/individuals found with BMI>30 should be barred towards the access of TJR/anthroplasty (Amen et al, 2006).
Winiarsky’s group performed a research on a population cohort with BMI>40 undergoing TJR. The result showed that 22% of the subjects suffered from wound complication, 10% individuals developed infection and 8% of the subjects suffered from ligament damage. When these result was compared with the wild type (normal population), it was seen that only 2% non-obese subjects developed wound complication, 0.6% suffered from infection and surprisingly there were non with ligament damage. Later, same group of individuals were studied after five years and significant post surgical differences were noticed in obese subjects as compared to the normal (non-obese) individuals. Therefore, we can conclude from the above study that obese patients have high risk during pre and post surgery (Vasqez et al, 2003). However, in Toronto, a random survey amongst 24231-population cohort showed that after 2-7 years of surgery there was a high level of patient satisfaction with reference to pain and function. In addition, there was no negative impact on outcome that co-related with subject’s age or obesity (Heisel et al, 2005).
In Los Angeles California, Miric et al studied several factors leading towards TJR complexity. Research was performed amongst 406 subjects undergoing total knee anthroplasty (TKA). According to the researchers, it was observed that there was a significant co-relation between BMI and subject’s cardiac history. Interestingly, patients with diabetes mellitus have had an increase stay in hospital as compared to the healthy (non-diabetic) patients. Therefore, the study concluded that there was not a significant difference amongst heavier patients as compared to those with normal BMI. In addition, the cut offs of BMI dividing overweight and obesity did not accurately divide patients into high/low risk categories (Foran et al, 2004).
In Scotland, research was performed amongst group of 283 TKA patients between 1995 and 1999 consisting of obese and non-obese subjects. Researchers concluded that there was no significant difference in complication rates (Peersman et al, 2001). In a similar study in Baltimore Maryland, evaluation outcome of TKA in 68 obese subjects showed that after five years of surgery there was no significant difference amongst obese and non-obese subjects. However, surprisingly after 7 years of surgery obese patients had a higher “implant failure” rate as compared to non-obese subjects. It was also noted that 12.3% of the obese patients had to go for a re-operation due to implant failure. In addition, deep vein thrombosis was only noticed in obese subjects. Pritchett and Bortel described that obese patients had greater blood loss and needed blood transfusion as well as longer operative time. Peersman supported the view saying that the increase in the infection rate in obese patients was due to the prolonged operative duration (Prichett and Bortel, 1991).
4. Obesity and Implant Failure As described in the previous section, in Baltimore, there was no evidence of either complication or mortality amongst obese patients after five years of surgery. However, the same group individuals suffered an “Implant Failure” after seven years of TJR. Various researches were conducted and scientists concluded that younger patients (age 65). Simulation of metal-onpolyethylene arthroplasty model under laboratory conditions showed that the principle cause of the device failure was due to increased wear rates when greater load was applied. Hence, it was proved that younger subjects due to their daily life routine were applying more force on the implant as compared to elder population cohort (Barbour et al, 1995; McKellop et al, 1995). Moreover, subjects who were able to reduce weight in seven years were living a healthier life as compared to other subjects. Therefore, we can conclude that obesity also potentially affects the device failure in long run.
5. Conclusion Recently, obesity and TJR has pulled the interest of several scientists, health care personals and even the government. Various government officials and trust group supporting financial aid are still under the impression that obesity leads to TJR. However, there is neither significant evidence nor sufficient clinical results to support their view. TJR surgery is a reliable procedure to offer sustainable pain relief and provide healthier life style regardless individual’s BMI. However, we cannot ignore the fact that obese individuals require special care in terms of patient handling, surgical exposure etc. In addition, obese subject are also at a high risk in wound healing, infection and longer duration of operative duration. It is also clinically proven that higher activity level leads towards device failure. As mentioned earlier, due to physical work restriction after TJR high probability lies towards increasing BMI. Therefore, it is recommended that individual’s should attend weight loss programme before undergoing TJR.
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