Secondary Logo

Journal Logo

Section II: Original Articles: Research

Effect of Estrogen on Cellular Metabolism of the Human Anterior Cruciate Ligament

Yu, Warren D. MD; Liu, Stephen H. MD; Hatch, Joshua D. MD; Panossian, Vahé MD; Finerman, Gerald A. M. MD

Author Information
Clinical Orthopaedics and Related Research: September 1999 - Volume 366 - Issue - p 229-238
  • Free

Abstract

Recent epidemiologic studies have recognized within various sports a significantly higher rate of anterior cruciate ligament injury in female athletes than in their male counterparts within various sports.12,27,41,44 Although the etiology of this phenomena remains unclear, possible explanations include gender differences in ligament or muscle strength, conditioning and endurance, anatomy, and training techniques.7,23,30,33 Unique to the female athlete is her exposure to constantly changing hormonal milieu throughout her reproductive years. For most of her life, the female athlete is exposed to a rhythmic variation in either endogenous hormones during a regular menstrual cycle or exogenous hormones via oral contraceptives. The subsequent fluctuations in the serum estrogen levels may in turn translate to changes in metabolism of cells in the anterior cruciate ligament. Realizing that estrogen can have individual or interactive effects on various metabolic processes, it is possible that estrogen also may have an influence on ligament structure, composition, and integrity.

A significant amount of data exist concerning the effects of estrogen on bone density and osteogenic cells15,37; however, few studies have explored the effect of the female sex hormones on ligamentous tissue. The identification of estrogen receptor positive fibroblasts in the human anterior cruciate ligament strongly suggests that female sex hormones may have an effect on the structure and composition of this ligament.20 The purpose of this study was to characterize the effects of estrogen on human anterior cruciate ligament fibroblast proliferation and procollagen synthesis within an in vitro system. It is hypothesized that there exists an inverse, dose dependent relationship between cellular metabolism (fibroblast proliferation and procollagen synthesis) and estrogen concentrations.

MATERIALS AND METHODS

Study Design

To compare the effects of exogenously applied estrogen on human anterior cruciate ligament fibroblasts, a primary cell culture was established using a modification of the explant culture technique by Nagineni et al.26 With exposure to various concentrations of estrogen, fibroblast proliferation and rate of collagen synthesis were monitored concurrently using 3H-thymidine incorporation and Types 1 and 3 procollagen specific equilibrium radioimmunoassays.

The cell line was obtained from an otherwise healthy, 32-year-old multiparous woman undergoing total knee replacement secondary to traumatic arthritis. The patient was a nonsmoker, and conservative management had failed for her arthritis. This investigation was approved by the institutional human subjects protection committee, and informed consent was obtained before enrollment.

Western Blotting

To test the efficacy and reliability of the procollagen monoclonal antibodies within a tissue culture system, western blot analysis was performed. Logarithmic dilutions of certified fetal calf serum and cell culture homogenates were compared with the radioimmunoassay standards for Types 1 and Type 3 procollagens. Specifically, media and cell lysates from Day 3 samples dosed with 0.025 ng/mL of estradiol were homogenized and centrifuged for 5 minutes at 15,000 g. Thereafter, the radioimmunoassay standards, logarithmic dilutions of certified 10% fetal calf serum (manufacturer specifications indicate 29 pg/mL of estradiol in stock fetal calf serum; Gibco Technologies Inc, Grand Island, NY), and the cell culture supernatant were run in a 10% gradient polyacrylimide gel for 2 hours. After transfer to nitrocellulose paper and blocking of nitrocellulose with nonfat milk, the primary antibody followed by the secondary immunofluorescent tagged antibody were applied separately for 1 hour each at room temperature. The relative banding patterns and intensities for Type 1 procollagen (Fig 1) and Type 3 procollagen were compared.

Fig 1
Fig 1:
Western blot analysis comparing antiprocollagen Type 1 antibody reactivity to radioimmunoassay standards for Type 1 procollagen, fetal calf serum, and Day 3 samples treated with 0.025 ng/mL of estradiol (E2). Antiprocollagen Type 1 antibody reactivity to fetal calf serum (arrows) is absent. RIA = radioimmunoassay.

Cell Culture Protocol

The anterior cruciate ligament of the subject was obtained under aseptic conditions in the operating room. The tissue was placed immediately into sterile tissue culture media. All additional manipulations were conducted under a sterile biologic hood. Four millimeters of the tissue adjacent to the bony insertion were discarded to avoid cartilaginous and periosteal contamination. The synovial sheath was excised. The remaining specimen was minced finely by transverse sections with a surgical blade and divided equally into four 75-cm2 tissue culture flasks (Corning Glass Works Inc, Corning, NY) containing 15 mL growth medium, which consisted of Minimal Essential Medium alpha (MEM-α) supplemented with certified 10% fetal calf serum (manufacturer specifications indicate 29 pg/mL of estradiol in stock fetal calf serum; Gibco Technologies Inc), gentamycin (5 μg/mL), and streptomycin (100 μg/mL).

Currently, estradiol free fetal calf serum is not available commercially. However, charcoal stripped fetal calf serum contains the lowest known concentration of estradiol available in the biotechnology market (at 0.0016 ng/mL of estradiol in 10% charcoal stripped fetal calf serum). The concentration of estradiol in charcoal stripped media was not significantly different than the 0.0029 ng/mL of estradiol present in 10% certified fetal calf serum. As such, it was opted to supplement the media with 10% certified fetal calf serum, which has been shown to contain a sufficient amount of growth factors to stimulate fibroblast proliferation optimally.27

Primary cell cultures were maintained in a humidified atmosphere of 5% CO2 and 95% air at 37° C without a change in media until 1 week after harvest. Thereafter, culture media was changed every 3 days. The growth of cells was monitored with an inverted phase contrast microscope. Once cells grew to 90% confluence, the explants were detached with 0.25% trypsin in calcium and magnesium free phosphate buffered saline (pH = 7.4) with 0.1 mmol ethylenediametetraacetic acid (Gibco Technologies Inc) for 5 minutes at 37°C. At this and subsequent subculturing, the cells were split at a 1:5 ratio.

Estrogen Administration

Fibroblasts cultured in the third passage to confluence were released with trypsin, washed in growth medium supplemented with 10% certified fetal calf serum, and cultured in 12-well Corning cluster plates at a density of approximately 5 × 104 cells per well. To show accurate experimental techniques, Group 1 was raised in serum free media. To simulate the physiologic and supraphysiologic levels of estradiol present in the human menstrual cycle at 0.025 to 0.3 ng/mL,31 experimental Groups 2 through 6 were supplemented with 10% certified fetal calf serum and received a near final logarithmic concentration of 17β-estradiol for 1, 3, 7, 10, and 14 days, as outlined in Table 1. The media with the appropriate concentrations of estrogen were replaced every 3 days.

TABLE 1
TABLE 1:
Tabulation of Final Logarithmic Concentrations of Estradiol Administered per Group

3H-Thymidine Incorporation

Deoxyribonucleic acid (DNA) content and DNA synthesis of control and experimental groups were monitored using 3H-thymidine incorporation of cultures.26 Confluent cultures at Passage 3, which had been exposed to the respective concentrations of estradiol for 1, 3, 7, 10, and 14 days, were incubated with 10 μL 3H-thymidine (1 μC/mL, New England Nuclear, Boston, MA) for 24 hours at 37°C in a humidity chamber at 5% CO2 and 95% air. Subsequently, agarose was precipitated and the free 3H-thymidine was removed. The remaining contents were transferred into glass test tubes containing 3 mL of 10% trichloroacetic acid. The contents of the tubes were vortexed and left on ice for 30 minutes to precipitate any macromolecules. Trichloroacetic acid perceptible radioactivity was collected by filtration through Whatman glass microfiber filters using a millipore filtration unit. Filters were air dried and placed into scintillation vials containing 10 mL of scintillation fluid (Liquiscint, National Diagnostics Inc, Manville, NJ, with 10% acetic acid), and the radioactivity was measured in a scintillation counter (Beckman 1801 Beta Counter, Beckman Inc, Pasadena, CA). Cell numbers in replicate of six wells were determined, and the thymidine incorporation was expressed in counts per minute/103 cells.

Quantification of Collagen Synthesis

The effects of varying concentrations of estradiol on procollagen levels, as an indicator of collagen synthesis, were determined using commercially available equilibrium radioimmunoassays. Levels of procollagen Type 1 were measured with the use of a monoclonal antibody directed against the trimeric carboxyterminal of procollagen 1 (PICP, Farmos Diagnostica, Turku, Finland),5,25 and procollagen Type 3 levels were measured by a monoclonal antibody specific for the aminoterminal of procollagen Type 3 (PIIINP, Farmos Diagnostica).32

Confluent cultures at Passage 3, which had been exposed to the respective concentrations of estradiol for 1, 3, 7, 10, and 14 days, were incubated with the respective 125I tracer and antibody for 2 hours at 37°C. Thereafter, the precipitating second antibody complex was added to separate the bound from the free tracer element. The tube was centrifuged and decanted after a 30-minute incubation at room temperature. The bound tracer in the pellet was placed into scintillation vials containing 10 mL of scintillation fluid (Liquiscint, National Diagnostics Inc, with 10% acetic acid), and the gamma counts were measured in a scintillation counter (Micromedic Systems 4/600 Automatic Gamma Counter, Micromedic Systems Inc, Horsham, PA). The resulting counts are inversely proportional to the amount of procollagen present in each sample. Cell numbers in replicate were determined, and thymidine incorporation was expressed as count per minute/103.

Statistical Analysis

Radiolabeled substrate incorporation was expressed as the mean ± standard error mean. Thereafter, data were analyzed using linear regression analysis. Statistical analysis of cellular proliferation, 3H-thymidine incorporation, and levels of procollagen Types 1 and 3 were performed by matched sign rank test. A significant level of 0.05 was selected and presented in the form of p values.

RESULTS

Western Blot

Western blot analysis of Type 1 and Type 3 procollagen radioimmunoassays showed numerous significant preliminary results that establish the scientific foundations of this study. The increased intensity of the banding pattern of the positive control radioimmunoassay standards for procollagens 1 and 3 proved the efficiency of the kits in recognizing increasing concentrations of the respective procollagens. In addition, as shown by the absence of any banding pattern when fetal calf serum was incubated with the antiType 1 procollagen antibodies (Fig 1, arrows), it was concluded that serum contains undetectable levels of procollagen. Similarly, Type 3 procollagen western blot analysis showed a banding pattern identical to that of the Type 1 procollagen. As a result, the quantitative measurements obtained through this assay reflect the actual level of procollagen produced by the human fibroblasts of the tissue culture system.

Fibroblast Proliferation

3H-thymidine incorporation is shown in Figure 2. On Days 1 and 3, there was a dose dependent decrease in the proliferation of anterior cruciate ligament fibroblasts with increasing estradiol concentrations (p < 0.01, R2 = 0.56; p < 0.01, R2 = 0.23 respectively; linear regression analysis). This dose dependent decrease in fibroblast proliferation with increasing estradiol concentrations became less apparent with time. By Days 7, 10, and 14, no statistically significant correlation was found between estrogen concentration and fibroblast proliferation.

Fig 2
Fig 2:
Histogram describing anterior cruciate ligament fibroblast proliferation as function of estradiol concentration and time.

Procollagen Synthesis

The effects of estradiol on Type 1 procollagen synthesis are shown in Figure 3. On Days 1 and 3, procollagen synthesis decreased in a dose dependent manner with increasing estradiol concentrations (p < 0.05, R2 = 0.33; p < 0.5, R2 = 0.13, respectively; linear regression analysis). Again, on Days 7, 10, and 14, this dose dependent decrease in Type 1 procollagen synthesis was attenuated, and no statistically significant relationship between estrogen concentrations and procollagen 1 synthesis was seen at these points.

Fig 3
Fig 3:
Histogram describing anterior cruciate ligament fibroblast Type 1 procollagen synthesis as a function of estradiol concentration and time.

The effects of estradiol on Type 3 procollagen synthesis are shown in Figure 4. No statistically significant differences in Type 3 procollagen synthesis were observed with varying estradiol concentrations at any time.

Fig 4
Fig 4:
Histogram describing anterior cruciate ligament fibroblast Type 3 procollagen synthesis as a function of estradiol concentration and time.

DISCUSSION

Collagen is produced by fibroblasts and organized in a hierarchical manner from fascicles to microfibrils, whereby it subsequently performs the major loadbearing function of the anterior cruciate ligament.13,36 Alterations in the metabolism of fibroblasts, by local stimuli with growth factors19 or systemic hormonal stimuli,16 have an influence on the quantity, type, and stability of the collagen in the anterior cruciate ligament.2,4,22,34

Collagen is synthesized as a larger molecule with propeptide extensions at both ends that are cleaved stoichiometrically during fibrinogenesis and released into the extracellular fluid. The sequence removed from the carboxyterminal end of the molecule, known as the carboxyterminal propeptide, can be found in the extracellular matrix. As a result, the extracellular levels of these cleaved extensions, which have been named as procollagens by the manufacturer of the equilibrium radioimmunoassays, directly reflect the extracellular levels of collagen. This stoichiometric proportionality has been supported by numerous studies in which levels of Type 1 and Type 3 procollagen were increased when collagen synthesis was stimulated by growth hormone therapy.4,38,40 Similarly, two studies of bone biopsy specimens from patients with various forms of metabolic bone disease29 and in postmenopausal women15 have reported that serum procollagen Type 1 levels correlate significantly with histomorphometric measure of bone formation and density.

It is known that female sex hormones have a widespread effect on the growth and development of bone, muscle, and connective tissues.18,35,43 Although numerous animal studies have documented the effect of estrogen on collagen metabolism in various tissues, ligaments have not been the subject of much investigation.3,7,10,11,13,35,43 Several in vivo studies have shown that the quantity of collagen in certain tissue is influenced by estrogen. Long term (in terms of weeks) estrogen administration causes a decrease in the total amount of collagen in the rat hip joint capsule, skin, aorta, and tail tendon.11,13,35 In contrast, two separate studies have documented an estrogen dose dependent increase in the production of Type 1 collagen messenger ribonucleic acid (mRNA) in osteoblasts.9,24

Many studies have shown how local estrogen concentrations may influence connective tissue metabolism. Recently, it has been reported that oral estrogen and progesterone therapy significantly reduced Type 1 procollagen in postmenopausal women.15 Equally, within a similar group of patients, there is an increase in total body Type 3 collagen content with daily administration of estrogen.14 Estrogen administration is known to decrease total collagen acutely in rat tendon and fascia and to decrease collagen synthesis acutely in rat periodontal tissue.8,10 In the rat uterus and primate sex skin, estrogen acutely increases newly synthesized collagen, mostly Types 1 and 3, while concurrently increasing collagen degradation, with the net result being a dynamic flux between the synthesis and the degradation of collagens.3,8 Although estrogen administration reduces stiffness in periarticular connective tissue of rabbit knees by approximately 50% compared with untreated animals,1 it also has been shown to decrease hexosamine and soluble collagen content.43 Thus, it stands to reason that acute fluctuations in the serum estrogen concentration during the menstrual cycle may induce changes in the metabolism, amount, and type of the collagen in the anterior cruciate ligament.

Previous work done in this laboratory has established the presence of estrogen receptors in the human anterior cruciate ligament.20 The identification of estrogen receptor positive fibroblasts in the human anterior cruciate ligament strongly suggests that female sex hormones may have an effect on the structure and composition of this ligament. In a recent study, a dose dependent decrease in cellular proliferation and total collagen synthesis was seen when rabbit anterior cruciate ligament fibroblasts were treated with estradiol for 2 weeks.21

Estrogen exerts a dose dependent effect on cellular metabolism. Similar to the mentioned preliminary results seen in the rabbit fibroblast, the results of the current in vitro study indicate that at physiologic levels of estradiol (0.025-0.25 ng/mL), there was a significant decrease in human fibroblast proliferation and specifically Type 1 procollagen levels. Eventually, the inhibitory effect of estradiol became asymptotic to the horizontal at the pharmacologic estradiol concentrations (2.5 and 25 ng/mL), indicative of maximal estradiol effect with the saturation of the estrogen receptor or downregulation of the receptor protein.

Regarding a temporal relationship, the effects of estradiol were most pronounced in the early periods of hormone exposure, Days 1 and 3. By Day 7, the dose dependent effects of estradiol on fibroblast proliferation (Fig 2) and Type 1 procollagen levels became attenuated. Again, this phenomenon with time may be attributable to normal cellular physiology of receptor saturation, cell contact inhibition, or more commonly, because of downregulation of the receptor protein.

In contrast to Type 1 procollagen, no significant changes were observed in Type 3 procollagen levels with varying doses of estradiol or with time. These results clearly show that Type 3 procollagen synthesis remains unaffected with estrogen exposure, indicating that presumed differences in fibroblast metabolism are manifested largely by changes in Type 1 procollagen synthesis, rather than Type 3.

The proper function of a ligament depends on the appropriate type, synthesis, assembly, crosslinking, and remodeling of collagen.6 This complex interplay between synthesis and remodeling of collagen is influenced by hormones, exercise, and immobilization.18,28,31,36,39,42 The amount of collagen bundles and the individual types of collagen influence the ability of the tendon to withstand loading. It also is known that Type 1 collagen imparts a great mechanical strength to connective tissues, whereas Type 3 collagen has been correlated directly with tissue elasticity.22 Although the relative strength of Type 1 collagen to Type 3 collagen remains unknown, it generally is agreed that a larger ratio of Type 1 to Type 3 collagen in the ligament is indicative of greater strength, whereas a lower ratio may be characteristic of tissue laxity.17 In applying this understanding to the outlined results, it follows that the relative decrease in Type 1 procollagen synthesis with increasing estradiol concentrations may translate to ligament weakening.

The observed causal interrelationship between acute changes in estradiol concentrations and procollagen synthesis continues to complement the clinical significance of these findings when one considers that female athletes are exposed to rhythmic variations in either endogenous hormones during a menstrual cycle or exogenous, nonphysiologic levels of hormones via the use of oral contraceptives. Taken together, these acute fluctuations in the serum estrogen levels translate to acute changes in fibroblast metabolism. The net effect of these cellular events can result in reduced strength of the ligament and subsequently lead to a higher risk of anterior cruciate ligament injury in female athletes.

The results clearly show that physiologic and nonphysiologic levels of estradiol have a significant dose dependent effect on the fibroblasts of the anterior cruciate ligament in the early period of hormone exposure. Fibroblast proliferation and Type 1 procollagen synthesis are reduced significantly with increasing estradiol concentrations. Thus, cumulative or acute fluctuations in serum estrogen concentrations, such as those occurring during the menstrual cycle or via the additive effect of exogenous estrogen in oral contraceptives, may induce changes in the metabolism of anterior cruciate ligament fibroblasts within this in vitro system. The resulting structural and compositional changes on the molecular level could result in decreased strength of the anterior cruciate ligament and predispose female athletes to ligament injury. Future studies in animal models will be needed to show estrogen induced mechanical weakening of the anterior cruciate ligament.

The rate of anterior cruciate ligament injury and causes of such injuries in women are distinct from those in men and deserve equal attention. Clearly, more research is needed to address the associative causative factors and the role of prevention in curtailing the rate of ligamentous injury in female athletes. The results of this study recognize the importance of female sex hormones, specifically estrogen, on collagen metabolism and provide a basis for future in vivo studies investigating the effects of estrogen on the biomechanical properties of the anterior cruciate ligament.

References

1. Akeson WH, Woo SL, Amiel D, Doty DH, Rutherford L: Value of 17-beta oestradiol in prevention of contracture formation. Ann Rheum Dis 35:429-436, 1975.
2. Attia ET, Bhargave MM, Warren RF, Hannafin JA: The effects of cyclic load on growth, matrix production and migration of tendon fibroblasts are cell density dependent. Trans Orthop Res Soc 21:2, 1996.
3. Bentley JP, Brenner RM, Linstedt AD, et al: Increased hyaluronate and collagen biosynthesis and fibroblast estrogen receptors in macaque sex skin. J Invest Dermatol 87:668-673, 1986.
4. Bugeson RE, Nimni ME: Collagen types. Molecular structure and tissue distribution. Clin Orthop 282:250-272, 1992.
5. Carey DE, Goldberg B, Ratzan SK, Rubin KR, Rowe DW: Radioimmunoassay for type I procollagen in growth hormone-deficient children before and during treatment with growth hormone. Pediatr Res 19:8-11, 1985.
6. Cooper RC, Misol S: Tendon and ligament insertions: A light and electron and microscopic study. J Bone Joint Surg 52A:1-20, 1992.
7. Costill D, Daniels J, Evans W, et al: Skeletal muscle enzymes and fiber composition in male and female track athletes. J Appl Physiol 40:149-154, 1976.
8. Dyer RF, Sodek J, Heersche JNM: The effect of 17beta-estradiol on collagen and noncollagenous protein synthesis in the uterus and some periodontal tissues. Endocrinology 107:1014-1021, 1980.
9. Ernst M, Heath JK, Rodan GA: Estradiol effects on proliferation, messenger ribonucleic acid for collagen and insulin-like growth factor-I, and parathyroid hormone-stimulated adenylate cyclase activity in osteoblastic cells from calvariae and long bones. Endocrinology 125:825-833, 1989.
10. Fischer GM: Comparison of collagen dynamics in different tissues under the influence of estradiol. Endocrinology 93:1216-1218, 1973.
11. Fischer GM, Swain ML: Effect of sex hormones on blood pressure and vascular connective tissue in castrated and noncastrated male rats. Am J Physiol 232:617-621, 1977.
12. Gray J, Taunton JE, MCKenzie DC, et al: A survey of injuries to the anterior cruciate ligament of the knee in female basketball players. Int J Sports Med 6:314-316, 1985.
13. Hama H, Yamamuro T, Takeda T: Experimental studies on connective tissue of the capsular ligament. Acta Orthop Scand 47:473-479, 1976.
14. Hassager C, Jensen LT, Podenphant J, Riis BJ, Christiansen C: Collagen synthesis in postmenopausal women therapy with anabolic steroid or female sex hormones. Metabolism 39:1167-1169, 1990.
15. Hassager C, Jensen LT, Johansen JS, et al: The carboxy-terminal propeptide of type I procollagen in serum as a marker of bone formation: The effect of nandrolone decanoate and female sex hormones. Metabolism 40:205-208, 1991.
16. Ho KKY, Weissberger AJ: Impact of short-term estrogen administration on growth hormone secretion and action: Distinct route-dependent effect on connective and bone tissue metabolism. J Bone Miner Res 7:821-927, 1992.
17. Laros GS, Tipton CM, Cooper RC: Influence of physical activity on ligament insertions in dogs. J Bone Joint Surg 53A:275-286, 1971.
18. Lebrun CM: The effect of the phase of the menstrual cycle and the birth control pill on athletic performance. Clin Sports Med 13:419-441, 1994.
19. Lee J, Green MH, Amiel D: Synergistic effect of growth factors on cell growth from explants of rabbit anterior cruciate and medial collateral ligaments. J Orthop Res 13:435-441, 1995.
20. Liu SH, Al-Shaikh R, Panossian V, et al: Primary immunolocalization of estrogen and progesterone target cells in the human anterior cruciate ligament. J Orthop Res 14:526-533, 1996.
21. Liu SH, Al-Shaikh, Panossian V, Finerman G, Lane JM: Estrogen affects the metabolism of the anterior cruciate ligament: A potential explanation for female athletic injury. Am J Sports Med 25:704-709, 1997.
22. Liu SH, Yang RS, Al-Shaikh R, Lane JM: Collagen in tendon, ligament, and bone healing. Clin Orthop 318:265-278, 1995.
23. MacDougall JD, Sale DG, Alway SE, Sutton JR: Differences in muscle fiber number in biceps brachii between males and females. Can J Appl Sport Sci 8:221-222, 1983.
24. Majeska RJ, Ryaby JT, Einhorn TA: Direct modulation of osteoblastic activity with estrogen. J Bone Joint Surg 76A:713-720, 1994.
25. Melkko J, Miemi S, Risteli L, Risteli J: Radioimmunoassay of the carboxy- terminal propeptide of human type I procollagen. Clin Chem 36:1328-1332, 1990.
26. Nagineni CN, Amiel D, Green MH, Berchuck M, Akeson WH: Characterization of the intrinsic properties of the anterior cruciate and medial collateral ligament cells: An in vitro cell culture study. J Orthop Res 10:465-475, 1992.
27. Nilsson S, Roaas A: Soccer injuries in adolescents. Am J Sports Med 6:358-361, 1978.
28. Noyes FR, Torvik PJ, Hyde WB, DeLucas JL: Biomechanics of ligament failure: An analysis of immobilization, exercise, and reconditioning effects in primates. J Bone Joint Surg 56A:1406-1418, 1974.
29. Parfit AM, Simon LS, Villanueva AR, Krane SM: Procollagen type I carboxy- terminal extension peptide in serum as a marker of collagen biosynthesis in bone. Correlation with iliac bone formation rates and comparison with total alkaline phosphatase. J Bone Miner Res 2:427-436, 1987.
30. Prince FP, Hikida RS, Hagerman FC: Muscle fiber types in women athletes vs non-athletes. Pflugers Arch 371:161-165, 1977.
31. Rebar RW: Practical Evaluations of Hormonal Status. In Yen S, Jaffe RB (eds). Reproductive Endocrinology: Physiology, Pathophysiology, and Clinical Management. Ed 2. Philadelphia, WB Saunders Company 683-733, 1986.
32. Risteli J, Niemi S, Trivedi P, et al: Rapid equilibrium radioimmunoassay for the amino-terminal propeptide of human type III procollagen. Clin Chem 34:715-718, 1988.
33. Schanz P, Randall-Fox E, Hutchison W, Tyden A, Astrand PO: Muscle fibre type distribution, muscle cross-sectional area and maximal voluntary strength in humans. Acta Physiol Scand 117:219-226, 1983.
34. Schmidt CC, Gergescu HI, Kwoh CK, et al: Effect of growth factors on the proliferation of fibroblasts from the medial collateral and anterior cruciate ligaments. J Orthop Res 13:184-190, 1995.
35. Shikata J, Sanada H, Yamamuro T, Takeda T: Experimental studies of the elastic fiber of the capsular ligament: Influence of ageing and sex hormones on the hip joint capsule of rats. Connect Tissue Res 7:21-27, 1979.
36. Smith BA, Livesay GA, Woo SL: Biology and biomechanics of the anterior cruciate ligament. Clin Sports Med 12:637-669, 1993.
37. Stevenson JC, Cust MP, Gangar KF, et al: Effects of transdermal vs oral hormone replacement therapy on bone density in spine and proximal femur in postmenopausal women. Lancet 336:265-269, 1991.
38. Tapanainen P, Risteli L, Knip M, Kaar ML, Risteli J: Serum amino-terminal propeptide of type II procollagen: A potential predictor of the response to growth hormone therapy. J Clin Endocrinol Metab 67:1244-1249, 1988.
39. Tipton CM, Matthes RD, Maynard JA, Carey RA: The influence of physical activity on ligaments and tendons. Med Sci Sport Exerc 7:165-175, 1975.
40. Trivedi P, Hindmarch P, Ristel J, et al: Growth velocity, growth hormone therapy and serum concentrations of the aminoterminal propeptide of type III procollagen. J Pediatr 114:225-230, 1989.
41. Whiteside PA: Men's and women's injuries in comparable sports. Phys Sports Med 8:130-136, 1980.
42. Woo SYL, Gomez M, Sites T, et al: The biomechanics and morphological changes in the medial collateral ligament of the rabbit after immobilization and remobilization. J Bone Joint Surg 69A:1200-1211, 1987.
43. Yamamuro T, Hama H, Takeda T, Shikata J, Sanada H: Biomechanical and hormonal factors in the etiology of congenital dislocation of the hip joint. Int Orthop 1:231-236, 1977.
44. Zelisko JA, Noble HB, Porter M: A comparison of men's and women's professional basketball injuries. Am J Sports Med 10:297-299, 1982.
© 1999 Lippincott Williams & Wilkins, Inc.