Bone and Lean Tissue Changes Following Cranial Cruciate Ligament Transection and Stifle Stabilization
Following cranial cruciate ligament transection and extracapsular stabilization, dual-energy X-ray absorptiometry was used to analyze bone mineral content and lean tissue mass in the surgical and nonsurgical legs (n=14) at 0, 2, 4, and 8 weeks, and to evaluate bone mineral content and bone mineral density (BMD) of the proximal, mid-, and distal tibia of both the surgical and nonsurgical legs (n=15) at 0, 5, and 10 weeks. There was significant loss of bone mineral content and lean tissue in the surgical leg compared to the nonsurgical leg. Significant loss in bone mineral content and BMD was detected in the tibia of the surgical leg and was most pronounced in the metaphyseal region.
Introduction
Cranial cruciate ligament injury is one of the most common orthopedic problems in dogs.1,2 After surgical repair, owners are generally instructed to restrict their dog’s activity.3 In addition, the animal is often unwilling to bear full weight on the operated limb for weeks to months. During this time of decreased activity, it is common to recognize muscle atrophy of the affected limb.4 Compensatory hypertrophy of the contralateral limb may also be observed when lameness results from an orthopedic condition.5 It is well documented that animals and humans experience muscle atrophy and bone loss with disuse.6–20 The information obtained from dogs has resulted from studies that use various limb immobilization techniques (e.g., casting, splinting, limb suspension, denervation).6–10
Mechanical load as well as growth factors, hormones, and vitamins affect growth and maintenance of bone.7,21 Bone is deposited in proportion to the compressional load that the bone must carry.22 Without normal weight-bearing, there is hypomineralization of cortical and cancellous bone in dogs.7 There is an increase in cancellous bone turnover and reduced density and stiffness of cortical bone.7 Bone loss occurs first in bone adjacent to marrow.10 Historically, radiographs have been used to evaluate bone quality, although as much as 40% to 50% loss of mineralization is required before changes can be detected radiographically.23
Many studies have been performed to evaluate the extent and cause of muscle atrophy with disuse.4,8,16–18,24 Loss of muscle mass has been documented and assessed by measuring the circumference of the area in question.25 More recently, ultrasonography, computed tomography, magnetic resonance imaging, and dual-energy X-ray absorptiometry (DEXA) have been used to estimate muscle mass.25
Dual-energy X-ray absorptiometry combines pencil or fan-beam X-ray absorptiometry with analytic computer software.26 This technique allows precise and accurate measurement of bone and soft-tissue body components.26,27 The X-ray beam is comprised of two energies that are attenuated differently, depending on the tissue type.26 The computer algorithm quantifies the various components. Bone is discriminated from soft tissue based on its differential attenuation.26 Fat tissue can be discriminated from lean tissue based on water content.26 The tissue mass is expressed in grams. Bone mineral density (BMD) is bone mineral content divided by the area computed to be bone and is expressed as grams per square centimeter.26 Bone mineral density is the most commonly reported DEXA value in humans; however, it requires making three-dimensional assumptions based on a two-dimensional image.26 Dogs are more varied in size, conformation, and bone geometry, which may make this calculation less accurate.26 Dual-energy X-ray absorptiometry is commonly used to evaluate body composition, particularly bone mineral content and BMD in humans.27–29 The accuracy and use of DEXA have been verified in clinically normal dogs.26 Serial use of DEXA on individuals or groups of individuals provides dynamic information with regard to changes in lean tissue mass and bone mass; therefore, changes after an injury and potential response to therapy can be more accurately documented.
Physical rehabilitation is becoming more common following surgical repair of cranial cruciate ligaments in dogs.30 To date, it has been directed almost solely at maintenance and restoration of soft tissue. Immobilization studies have shown that loss of bone mineral content can also be an important process with decreased use of a limb.6,7,9 Knowledge of the timing and extent of these changes may allow more specific strategies to be developed that prevent or attenuate bone and lean tissue loss following surgery. No study to date has focused on the degree of lean tissue and bone loss that occurs following cranial cruciate ligament injury and surgery in dogs.
The present study was performed in two parts. Two groups of dogs were used to study different but related changes in bone following cranial cruciate ligament transection and stabilization. By documenting changes in two different groups of dogs, evidence of consistency would be provided. The goal of the first part of the study was to determine changes, if any, in bone mineral content and lean tissue in both the affected and contralateral limbs during the immediate 8 weeks following cranial cruciate ligament transection and stifle stabilization. Based on the results obtained in part 1, the goal of part 2, which was performed in another group of dogs, was to evaluate for differential bone loss between the metaphysis and diaphysis of the tibia in the immediate 10-week postoperative period. It was hypothesized (part 1) that, with the use of DEXA, there would be demonstrable loss of both bone and lean tissue mass from the surgical limb, as well as a mild compensatory muscle hypertrophy and increased bone mineral content in the contralateral limb. Further, it was hypothesized (part 2) that the metaphyseal region of the tibia would lose more bone mass than the diaphysis.
Materials and Methods
Study Subjects
In part 1 of the study, 14 adult (2 to 5 years of age), mixed-breed, dedicated research dogs were used. In part 2 of the study, 15 adult (2 to 5 years of age), mixed-breed, dedicated research dogs were used. All dogs were housed under standard laboratory conditions and were fed a standard balanced and complete canine diet.a Housing kennels measured 1.22 m × 2.13 m. The dogs were maintained on a constant day:night cycle (12 hour:12 hour). Each animal was determined to be healthy on the basis of normal physical and orthopedic examination findings, as well as a normal complete blood count, serum biochemical analysis, and urinalysis. Radiographs were taken of the hips and rear legs to ensure that no undiagnosed orthopedic conditions were present. Prior to the start of the study, all dogs were socialized and leash walked for approximately 10 minutes twice daily for a minimum of 2 weeks. The Institutional Animal Care and Use Committee approved all protocols.
Surgery
All dogs were premedicated with an intramuscular injection of acepromazine maleateb (0.1 mg/kg), butorphanol tartratec (0.4 mg/kg), and glycopyrolated (0.01 mg/kg). Propofole (4 mg/kg) was administered intravenously to effect for induction of anesthesia. A surgical plane of anesthesia was maintained on isofluranef in oxygen. The dogs were randomly assigned to have surgery on either the right or left leg. An arthrotomy was performed, and the designated cranial cruciate ligament was transected near its proximal and distal attachments. Cranial drawer was confirmed. Two strands of 50-lb nylon suture were passed around the lateral fabella and through a preformed hole in the tibial crest.31 These sutures were tightened manually and secured with tube crimpsg placed laterally. Routine closure of the incision was performed. Butorphanol tartrate (0.4 mg/kg) was administered intravenously immediately after surgery.
Postoperative Care
Deracoxibh (3 mg/kg per day) was administered orally for 4 days postoperatively. Each dog was leash walked twice daily starting at the beginning of the second postoperative week. Dogs were initially walked for 10 minutes twice daily, and this was incrementally increased by 5 minutes per day, every week to a maximum of 20 minutes twice daily by week 6 and thereafter. Daily passive range of motion exercises (15 to 20 repetitions) were performed on the surgical leg for the first 3 weeks postoperatively.
Dual-Energy X-ray Absorptiometry
Dual-energy X-ray absorptiometry was performed at specified times after surgery. For the DEXA scans, the animals were anesthetized using the same protocol as used for the surgery. They were positioned on the scanning table in dorsal recumbency, with the hind legs extended and the forelegs positioned along the side of the thorax and cranial abdomen. The legs were secured with 1-inch tape to maintain body symmetry. Total-body absorptiometry scansi were performed of the entire pelvic limbs. For the animals in part 1, the information from the scans was used to determine bone mineral content and lean tissue mass of both hind limbs before surgery (week 0) and at 2, 4, and 8 weeks after surgery. For part 2, data was acquired from 5-mm slices over the proximal, mid-, and distal tibia of both pelvic limbs. The location of the proximal tibial slice was determined by measuring 5 mm distal to the stifle joint on a craniocaudal scan view. The distal tibial slice was taken 5 mm proximal to the tibiotarsal joint. The midtibial slice was acquired at the location where the fibula diverged from the tibia on the craniocaudal scan image. Data from these slices were analyzed to determine bone mineral content and BMD before surgery (week 0) and at 5 and 10 weeks after surgery. Based on the information obtained in part 1, slightly different time intervals were chosen for data collection in part 2 to further characterize changes occurring in bone after the surgery.
Statistical Analysis
One-way analysis of variance (ANOVA) was used to determine significant differences between the surgical and nonsurgical legs. When differences were found, the Tukey test (multiple comparison procedures) was used to evaluate the differences. A student’s t-test was used to determine if there was a significant difference between males and females for bone mineral content or lean tissue at week 0.j Significance was set at P<0.05.
Results
Part 1
Seven intact males and seven intact females were used in this portion of the study. Mean body weight (± standard deviation) was 21.17±1.52 kg. The results of the bone mineral content and lean tissue data of the surgical and nonsurgical legs at weeks 0, 2, 4, and 8 are shown in Figures 1 and 2, respectively. No differences were detected between the surgical and nonsurgical legs for either bone mineral content or lean tissue at week 0. There were also no differences in baseline values between males and females. Significant differences did occur in the bone mineral content (P<0.001) and lean tissue (P<0.001) between the surgical and nonsurgical legs over the 8-week study period. Significant differences in bone mineral content were detected between week 0 and week 8 for the surgical leg (P=0.031) and between the surgical and nonsurgical legs at 8 weeks (P<0.001). For lean tissue, significant differences between the surgical and nonsurgical legs occurred at 4 weeks (P=0.008) and 8 weeks (P=0.002). There was also a significant difference in lean tissue between weeks 0 and 4 in the surgical leg (P=0.002).
The bone mineral content of the surgical leg remained constant for 2 weeks postsurgery and then declined at weeks 4 and 8. By week 8, the bone mineral content of the surgical leg was 15.6% below the baseline (week 0) value. The bone mineral content of the nonsurgical limb remained constant until week 4, and then it increased. By week 8, the bone mineral content of the nonsurgical leg was 3.1% above the baseline value.
The lean tissue mass of the surgical leg declined until week 4, and then it began to return toward baseline. At week 4, the lean tissue mass of the surgical leg was 16.0% lower than baseline, and it rebounded to 11.7% below the baseline measurement by week 8. The lean tissue mass of the nonsurgical leg remained constant until week 4, and then it began to rise. At week 8, the lean tissue mass was 2.6% above the baseline value.
Part 2
Seven intact males and eight intact females were used in this part of the study. Mean body weight for the dogs was 22.55±3.03 kg. The results of bone mineral content analysis of the proximal, mid-, and distal tibia in both the surgical leg and nonsurgical leg are illustrated in Figures 3A, 3B, and 3C. Significant differences in bone mineral content of the proximal (P=0.05) and distal (P=0.013) tibia were found between the surgical and nonsurgical legs over the time of the study. The bone mineral content of the surgical leg decreased by 15.3% in the proximal tibia and by 14.8% in the distal tibia at 10 weeks. The bone mineral content of the nonsurgical leg increased by 2.6% and 4.1% in the proximal and distal tibia, respectively, during the same time period. The bone mineral content in the mid-diaphysis of the surgical leg was lower at week 10 than at week 0, but this decrease was not significant.
The results of the BMD analysis of the proximal, mid-, and distal tibia in both the surgical and nonsurgical legs are shown in Figures 4A, 4B, and 4C. Significant differences occurred in BMD at all three sites over the study period (i.e., proximal tibia, P<0.001; midtibia, P=0.01; distal tibia, P<0.001). Significant differences were detected between the surgical and nonsurgical legs at week 10 for both the proximal (P<0.001) and distal (P<0.001) tibia. There was also a significant difference between the surgical and nonsurgical legs for the distal tibia at week 5 (P<0.001). Significant differences in the surgical leg occurred in the proximal tibia between weeks 0 and 10 (P<0.001), and in the distal tibia between weeks 0 and 5 (P=0.002) and between weeks 0 and 10 (P=0.002). The BMD of the surgical leg decreased by 17.9%, 8.3%, and 12.5% in the proximal, mid-, and distal tibia, respectively, between weeks 0 and 10. Over the same time period, the nonsurgical leg BMD increased by 3.6%, 2.8%, and 4.2% in the proximal, mid-, and distal tibia, respectively.
Discussion
Cranial cruciate ligament injuries are very common, especially in medium- and large-breed dogs.1 Regardless of the mode of therapy chosen to stabilize the stifle, activity is usually restricted postinjury and postoperatively.3 The animal often restricts its own weight bearing because of the discomfort of joint inflammation and surgical pain. The veterinarian generally recommends minimal and controlled exercise for 8 to 12 weeks after injury and surgical stabilization.3 Decreased weight bearing and restricted activity result in loss of both bone and muscle. Physical rehabilitation is now more commonly recommended and used in small animal practice.30 The goals of therapeutic exercise are to improve or maintain strength, mobility, flexibility, and function.32 In general, therapeutic exercise is directed toward muscle strength and joint mobility. Very little attention has been given specifically to the problem of bone loss.
Following surgery to stabilize a transected cranial cruciate ligament, the patterns for tissue loss and recovery apparently differ between muscle and bone. Attempts were made in the study reported here to mimic the clinical course of an otherwise healthy dog with an acute rupture of the cranial cruciate ligament. The animals were confined, and exercise was restricted in a fashion similar to what is commonly recommended for dogs not undergoing intense physical rehabilitation. Results of this study revealed muscle atrophy of the surgical leg by 2 weeks, with muscle mass beginning to return after 4 weeks. The observed bone loss in the surgical leg lagged behind the muscle loss by 2 weeks and did not show any sign of recovery by the end of the study (week 8). These findings were similar to those in humans.19,33 One study involving stroke patients with hemiparesis reported that lean muscle mass was rapidly lost after insult, but it was also rapidly regained with weight bearing and use.19 Conversely, preservation or restoration of bone mineral content was difficult, but less bone loss occurred in those patients that returned to early ambulation.19 In a separate study, 44 humans with ligamentous knee injuries were evaluated for bone loss.33 In the people that required surgery, there was an 18% average loss of bone mineral content and no signs of mineral content restoration during the first year.33
It is accepted that fractures in small animals can occur secondary to bone loss caused by bony neoplasms or metabolic bone disease.34 Significant research in humans has been directed toward reducing bone loss secondary to decreased use, hormonal and metabolic influences, aging, and injuries.27 Less attention has been given to the potential detrimental effects of bone loss in animals. Decreased BMD or bone mineral content is rarely considered a contributing factor in canine fractures unless there is radiographic evidence of osteopenia. Because a significant amount of bone must be lost to be radiographically detectable, bone loss may play a more important role in canine fractures than previously thought. Studies utilizing DEXA analysis for comparing parameters in animals with fractures, and in age- and conformation-matched controls, are required to determine if a link exists between fractures and decreased BMD or bone mineral content.
Although the study reported here was designed to mimic an acute cranial cruciate rupture in a healthy, young adult dog, it is important to acknowledge that the study conditions did not fit the profile of all dogs with this injury. Some affected animals may present with preexisting bone loss. Many animals present with chronic lameness and (based on the results of this study) may already have significant muscle and bone loss that should be considered when planning surgical stabilization and postoperative care. The age of the animal must also be considered, as it has been shown that older dogs may lose proportionately less bone with disuse, but their residual deficits are greater.9 Active dogs are likely to have more bone mass and bone mineral content than inactive dogs. This assumption is based on human studies that show that increased dynamic load increases bone mass.35 Therefore, injury-related bone loss in active dogs may be less significant. Any concurrent disease process or drug therapy that may interfere with normal bone quality must also be taken into consideration when planning an orthopedic repair and postoperative rehabilitation program.
Significant differences in bone mineral content were detected when comparing the surgical and nonsurgical legs in this study. This finding was in agreement with a previous study where bone mineral contents of the surgical and nonsurgical legs were compared after creation and repair of a proximal femoral physeal fracture in dogs.36 In that study, there was a significant difference found in the bone mineral content of the entire proximal femur of the surgical leg compared to the opposite side at 4 and 8 weeks.36 Based on these findings, rehabilitation measures and postoperative therapies should attempt to address this problem of bone loss, especially in an animal with other risk factors for decreased bone mineral content. It has been shown that improved muscle strength can improve bone density or decrease bone loss; therefore, aggressive postoperative rehabilitation therapy may have a positive influence on bone loss.27,37 Pharmaceuticals that have been experimentally shown to increase bone mass or decrease loss in dogs include bisphos-phonate pamidronate, alendronate, dihydroxycholecalciferol, and prostaglandin E inhibitors.6,7,38,39 Extracorporeal shock-wave therapy has also been shown to increase bone production in animal fracture models and in humans.40–46 The potential use of such agents in rehabilitation as a means of preventing bone loss and stimulating bone production requires investigation.
Sectional analyses of the tibia (part 2) confirmed bone loss in the surgical leg and a modest compensatory increase in the nonsurgical leg. These changes were most apparent in the metaphyseal regions. The different intervals for analysis (5 and 10 weeks in part 2 versus 4 and 8 weeks in part 1) were chosen, because in part 1 there was no significant loss of bone before 4 weeks, and the bone mineral content continued to decline even at 8 weeks. The duration of part 2 was extended in an attempt to analyze the bone mineral content over a longer period of time.
No significant change occurred in the bone mineral content in the mid-diaphysis of the tibia, but a change was found in BMD. This difference may reflect the fact that calculations for BMD for a three-dimensional subject are based on data obtained from a two-dimensional image, so an overestimation of change in BMD can occur if there are minimal changes in two-dimensional bone area. Until further experience is gained in the calculations of BMD in animals, the bone mineral content data may be more reliable. The metaphysis has the largest surface area and has more trabecular bone, which may facilitate increased bone resorption.5 The findings in the present study agreed with a study involving humans with femoral neck fractures.5 In the human study, bone loss occurred in the distal femoral metaphysis and proximal tibial metaphysis, but no cortical bone loss was found in the mid-femur, suggesting that cancellous bone is more sensitive to osteopenia.5 Fractures in humans with osteoporosis also occur most often in the metaphysis.27
In the study reported here, the operated stifle was stabilized using a common extracapsular technique.31 The tibial plateau leveling osteotomy (TPLO) procedure is also often used to stabilize a cranial cruciate deficient stifle, particularly in large dogs. The TPLO is thought to promote earlier weight bearing than other methods, which may have a positive effect on the amount of postoperative bone loss.47 It would be helpful to perform a similar study to the one reported here on dogs undergoing a TPLO, to identify any postoperative differences in bone and lean tissue mass between the two techniques.
Conclusion
Dual-energy X-ray absorptiometry analysis confirmed significant loss of bone mineral content, BMD, and lean tissue mass in dog legs following cranial cruciate ligament transection and stabilization. The rate of loss and regeneration differs between bone and lean tissue. Knowledge of this pattern may help with the development of more effective rehabilitation strategies. Further research is needed regarding the type, intensity, and duration of exercise or loading on bone required to inhibit or minimize bone loss. The effect of rehabilitation therapy and the potential use of pharmacological agents on bone loss during the postoperative period also require further investigation.
Global 21% Protein Dog Diet; Harlan Teklad, Madison, WI 53744-4220
Acepromazine maleate; Boehringer Ingelheim, St. Joseph, MO 64506
Butorphanol Tartrate; Fort Dodge Animal Health, Fort Dodge, IA 50501
Glycopyrrolate; American Regent, Inc., Shirley, NY 11967
Propoflo; Abbott Laboratories, North Chicago, IL 60064
Isoflurane; Abbott Laboratories, North Chicago, IL 60064
10-mm Crimp Tube; Jorgensen Laboratories, Inc., Loveland, CO 80538
Deramaxx; Novartis Health Animal US, Inc., Greensboro, NC 27408
Lunar model DPX; Lunar Corp., Madison, WI 53713
Sigma Stat; Chicago, IL 60611
Citation: Journal of the American Animal Hospital Association 42, 2; 10.5326/0420127
Citation: Journal of the American Animal Hospital Association 42, 2; 10.5326/0420127
Citation: Journal of the American Animal Hospital Association 42, 2; 10.5326/0420127
Citation: Journal of the American Animal Hospital Association 42, 2; 10.5326/0420127
Changes in mean bone mineral content (g) over time for the surgical and nonsurgical legs. A significant difference in bone mineral content (P<0.001) occurred between the surgical and nonsurgical legs over the study period. Significant differences were found (P=0.031) between weeks 0 and 8 for the surgical leg and between the surgical and nonsurgical (P<0.001) legs at week 8. Error bars indicate the standard error of the mean (SEM). Sx Leg=surgical leg; Non-Sx Leg=nonsurgical leg.
Changes in mean lean tissue content (g) over time for the surgical and nonsurgical legs. A significant difference in lean tissue mass (P<0.001) occurred between the surgical and nonsurgical legs over the study period. Significant differences were found (P=0.002) between weeks 0 and 4 for the surgical leg and between the surgical and nonsurgical legs at weeks 4 (P=0.008) and 8 (P=0.002). Error bars indicate the standard error of the mean (SEM). Sx Leg=surgical leg; Non-Sx Leg=nonsurgical leg.
Changes in mean bone mineral content (g) over time at the proximal tibial metaphysis (A), midtibial diaphysis (B), and distal tibial metaphysis (C) of the surgical and nonsurgical legs. Significant differences in the proximal (P=0.05) and distal (P=0.013) tibias occurred between the surgical and nonsurgical legs over the study period. There was no difference in bone mineral content of the midtibia of the surgical and nonsurgical legs. Error bars indicate the standard error of the mean (SEM). Sx Leg=surgical leg; Non-Sx Leg=nonsurgical leg.
Changes in mean bone mineral density (g/cm2) over time at the proximal tibial metaphysis (A), midtibial diaphysis (B), and distal tibial metaphysis (C) of the surgical and nonsurgical legs. Significant differences in BMD occurred at the proximal (P<0.001), mid- (P=0.01), and distal (P<0.001) tibia over the study period. Significant differences were found between the surgical and nonsurgical legs for the proximal tibia at week 10 (P<0.001) and for the distal tibia at both week 5 (P<0.001) and week 10 (P<0.001). Significant differences were detected in the surgical leg at the proximal tibia (P<0.001) between weeks 0 and 10, and in the distal tibia between weeks 0 and 5 (P=0.002) and between weeks 0 and 10 (P=0.002). Error bars indicate the standard error of the mean (SEM). Sx Leg=surgical leg; Non-Sx Leg=nonsurgical leg.
