Patellar stress fracture CAROL C. TEITZ,* MD, AND RICHARD M. HARRINGTON, MS From the
Department of Orthopaedics, University of Washington, Seattle, Washington
Stress fracture of the patella is a rare injury. It was first described in the English literature by Devas in 1960,’ but previously noted by Muller in 1943.19 Devas described one longitudinal and one transverse stress fracture, as well as
transverse, minimally displaced, distal third patellar fracture
symptomatic bipartite patella suggesting stress fracture,
trochlear surface of the femur. The fracture was noncomminuted and the trabecular bone at the fracture site appeared sclerotic, both proximally and distally. The fracture was anatomically reduced and internal fixation using tension band wiring was performed (Fig. 2). The knee was placed in an immobilizer and the patient was allowed to bear weight as tolerated. On the 3rd postoperative day, he began active-assisted range of motion knee exercises. One week postoperatively he had no swelling and 90° of flexion. Two weeks postoperatively the patient was in a motor vehicle accident and braced himself for the impact using both arms and both legs. His knees did not contact the dashboard; however, he developed a moderate effusion in the previously injured knee. He was examined the following day and had no ecchymosis or lacerations. Radiographs taken at that time revealed widening of the fracture site. A second open reduction and internal fixation was performed using Kirschner wires, a tension band, and a malleolar screw. The patient began active-assisted knee motion exercises on the 2nd postoperative day. His subsequent course was unremarkable. By 6 weeks after surgery, the patient was walking without a limp. His knee moved from 0° to 100° of flexion and he had no extensor lag. Additional quadriceps rehabilitation and range of motion exercises were
one
(Fig. 1). At the time of surgery the patellar retinaculum was found to be intact and there was no evidence of contusion on the
in three male athletes aged 16, 23, and 28.’ Since that time, patellar stress fractures have been described in two groups of people, patients with cerebral palsy’, 17,18,25 and adolescent athletes.’ 9’z ls zs In the athletic group, patellar stress fractures have been reported in six boys aged 11 to 179 lz 16 and in one girl aged 10.16 These fractures included three longitudinal and four transverse stress fractures. The transverse fractures occurred during basketball, soccer, and high jump-
ing. The cases presented herein occurred during sports that different from those previously reported and may therefore have a distinct biomechanical etiology. One occurred during sailboarding and the other during belly dancing. Both activities require prolonged knee flexion and quadriceps contraction. The second case discussed here is the first report of patellar stress fracture in a woman and in a patient more than 30 years old. are
CASE REPORTS Case 1 A 23-year-old male ski instructor presented with acute right knee pain. He gave a history of several months of knee pain, suggestive of patellar tendinitis. This had not interfered with his skiing activities. As summer approached, he had begun sailboarding on a daily basis. While sailboarding on the day before presentation, he felt a sudden crack in his right knee. He was then unable to lift his right leg or bear weight on it. Physical examination of the patient’s right knee revealed a tense effusion and tenderness over the dorsum of the patella. The remainder of his knee examination was normal. Eighty milliliters of blood in which fat droplets were visible were aspirated from his right knee. Radiographs revealed a *
Address correspondence and repnnt requests 15, University of Washington, Seattle, WA 98195
to Carol C
prescribed. Six months later the patient was skiing without problems, but began to have symptoms related to the Kirschner wires’ subcutaneous position (Fig. 3). Approximately 1 year after the original fracture he underwent hardware removal. Six months later he worked as a ski instructor. Physical examination 20 months after the original injury revealed full range of motion, 0.5 inch of vastus medialis atrophy, and mild patellofemoral crepitus. Three and one-half years following fracture, the patient reported that his right knee was &dquo;excellent.&dquo; He denied swelling or pain, but noted mild residual weakness, such as inability to do a one-legged deep knee bend. He was skiing vigorously without problems.
Teitz, MD, GB761
762
Case 2 A
36-year-old woman presented with right knee pain. She reported a vague aching in her right knee for approximately 1 month before presentation. She stated that the cause of the pain was related to belly dancing, which she practiced approximately 2 to 3 hours per day and performed on weekends, 2 to 3 hours at a time. Three days before presentation, the patient was dancing when she felt a pop in her right knee and was unable to bear weight on that leg. She noted moderate swelling the following morning. Physical examination of the right knee revealed a small effusion and tenderness to palpation over the distal portion of the patella. There was no evidence of other tenderness or ligamentous laxity. Radiographic examination revealed a nondisplaced fracture of the distal third of the patella (Fig. 4). The patient’s knee was placed in a long leg cast for 4 weeks. At that time the patella was nontender and radiographs revealed fracture healing (Fig. 5). Quadriceps rehabilitation and range of motion exercises were initiated. When last seen 3 months after her injury, she was doing well and had no complaints. She had full range of motion, no patellofemoral crepitus, and had equal quadriceps strength in both legs.
DISCUSSION
Figure 2. A lateral radiograph taken after tension band wiring.
Because the patella lies within the quadriceps mechanism, the patellofemoral compression force is the resultant of the
Figure 3.
The patient’s right knee 1 year after internal fixation, before hardware removal.
secondary
763
patellar tendon force vector and the quadriceps force vector24 (Fig. 6). When the knee is flexed, a bending stress is applied to the patella, which is loaded in tension on its superficial surface and in compression on its deep surface. This recurrent bending stress produces in the patella a specific trabecular architecture that has been shown by quantitative steriologic analysis to correspond to the architecture predicted by finite element analysis.13,2’ Using a combination of contact microradiographs and finite element analysis of the patella, Hayes and associates&dquo; found that trabecular orientation conformed to principal stress directions, and trabecular density was related to the magnitude of the imposed stresses. Hayes and Snyder14 subsequently showed that the maximal principal stresses in the patella were tensile and that large tensile stresses (maximum, 8.5 MPa) were induced in the anterior patella by the combined action of the quadriceps and patellar tendons. This finding corresponded to the trabecular orientation in
4. A lateral radiograph of the second knee at initial presentation.
Figure
Figure 5. Four weeks after presentation, graph was taken.
patient’s right
this lateral radio-
Figure 6. Tensile forces are applied to the anterior surface of the patella through the quadnceps tendon (Fi) and the patellar tendon (F2). The posterior surface of the patella is loaded in compression. The patellofemoral compression force (R) is a function of F1 and F2 as well as of the angle of flexion of the knee.
764
the anterior patella, which is predominantly proximal to distal. 23 The subchondral region of the patella experienced maximal compressive stresses of 3 MPa. They concluded that the resultant stresses in the patella are similar to those in a short beam subjected to three-point bending with superimposed axial tension. The patellofemoral joint reaction force must be equal and opposite to the patellofemoral compression force and changes as a function of both the angle of flexion of the knee and the magnitude of quadriceps force, which increased 6% per degree of flexion. Reilly and Martens24 found that the patellofemoral joint reaction force was at least 0.5 times body weight during level walking, 3.3 times body weight going up and down stairs, and 7.6 times body weight during a deep knee bend. Huberti and Hayes15 estimated maximal patellofemoral contact forces of approximately 6.5 times body weight when the knee was flexed 90°. Perry et al.2z found that during stance, when the knee was in 30° of flexion, the quadriceps force required to stabilize the knee equaled 210% of body weight. They also found that this quadriceps force equaled 50% of the average maximum quadriceps strength. (In a 70-kg person, the quadriceps tension was measured at 1375 N when the knee was flexed to 30°.) These large quadriceps forces, markedly increased at 30° of flexion, enable us to proffer a mechanism whereby bending moments could produce a fatigue fracture of the patella initiated in tension, without requiring sudden violent muscle contraction or sudden knee flexion as in previously described cases. In addition, time-dependent damage may be caused by creep in addition to or rather than fatigue. The total damage in a living, self-healing structure at any moment in time is the summation of damage caused by disease and aging plus the damage caused by mechanical stress, reduced by the damage repaired by healing processes. In human bone, the damage caused by mechanical stress is the summation of damage caused by cyclically applied loading (fatigue damage) and damage caused by steady application of load (creep damage). Carter and Hayes’ conducted cycle-dependent fatigue tests on human bone at physiologic temperature and loading rates. The tests showed that bone fatigue damage is diffuse in the tissue and causes a progressive decrease in strength and stiffness. Carter and Caler;J also conducted time-dependent creep fracture tests on human bone. The steady application of load resulted in the progressive accumulation of cracks within the material, with such accumulation dependent on both magnitude and duration of loading. Patellar fractures caused by &dquo;indirect force&dquo; have been described previously, 11 20 although their incidence in a large study of 707 patellar fractures was 1.5%21 and there was none in another study of 422 patellar fractures.’ In these reports, patellar fractures not caused by a direct blow were thought to be caused by sudden, violent quadriceps contraction or by high tensile forces created by the quadriceps muscle and patellar ligament. In all cases, the fractures were associated with extensive rupture of the patellar retinaculum.
Stress fractures generally occur because of high repetitions of loads that are below the yield strength of a given material.’ In living bone, fatigue fracture is not merely a function of cyclical loading but also depends on the frequency of loading.l° Fatigue fracture results when the frequency of loading precludes the remodeling necessary to prevent failure. Hence, it is the result of the amount and duration of load in concert with the degree of reparative ability. Muscle strength can also affect the likelihood of stress fracture, either by shielding the bone from applied stresses’ or by loading the bone as a function of increasing muscle contraction. The latter applies to the patella, which is loaded as a function of quadriceps force. Walter and Wolf&dquo; felt that athletes could repetitively load bone close to its yield strength because of superior muscle tone. In the cases previously described by Hanel and Burdge, 12 Sugiura et al.,26 and Dickason and Fox,9 the transverse patellar fractures occurred when the knee was in a flexed position and a sudden forceful contraction occurred in the process of jumping or kicking. Whether previous submaximal tensile loads had produced a weakened area in the patella is unknown and we have no history of previous complaints in these patients. The transverse fracture previously described by Devas’ occurred in a man who ran frequently and who theoretically required no explosive burst of quadriceps power. Patellar stress fractures also have been described in cerebral palsy patients with spastic lower extremities, many of whom had knee flexion contractures. 2,17 18 25 In both cases presented here, the patients’ activities required prolonged isometric quadriceps contraction with the knee in approximately 30° to 45° of flexion. All of the cases mentioned here may represent stress fractures with slightly different characteristics. In the kickers and jumpers the loads may have been of high magnitude, thereby requiring few cycles to fatigue. This theory is corroborated by the soft tissue damage noted in the retinaculum in these previously described patients. In cerebral palsy patients, the loads may be of varying magnitude and occur tonically or clonically. In our patients with no soft tissue damage, the positions and quadriceps contractions required by their activities may have produced a fracture because of creep or because of fatigue from relatively low loads cycled often enough to prevent repair.
CONCLUSIONS Two
presented of transverse patellar stress fracpatients whose activities required prolonged isometric quadriceps contraction and relatively constant knee cases are
tures in
flexion. We theorize that these stress fractures occurred because of repeated bending moments (possibly in association with creep) that initiated a fracture in tension on the superficial surface of the patella. Stress fractures on the tensile surface in other bones (e.g., femoral neck) are more likely to proceed to complete fracture, or heal more poorly than those on the compressive surface.l° Patients whose
765
activities require frequent knee flexion and quadriceps use, either tonically or in sudden bursts, are at risk for these fractures. Such patients presenting with knee pain and superficial patellar tenderness should undergo radiographic examination and possibly bone scan to identify early stress fractures of the patella before they become complete fractures.
REFERENCES
12
13 14
15
16 17 18
1
2 3 4
5 6
Brostrom A Fracture of the patella A study of 422 patellar fractures Acta Orthop Scand (Suppl) 143 1-80, 1972 Cahuzac M, Nichil J, Olle R, et al Les fractures de fatigue de la rotule chez l’infirme moteur de’origine cerebrale Rev Chir Orthop 79 87, 1965 Carter DR, Caler WE A cumulative damage model for bone fracture J Orthop Res 3 84-90, 1985 Carter DR, Hayes WC Compact bone fatigue damage A microscopic examination Clin Orthop 127 265-274, 1977 Chamay A Mechanical and morphological aspects of experimental overload and fatigue in bone J Biomech 3 263-270, 1970 Clement DB Tibial stress syndrome in athletes Am J Sports Med 2 81-
19
20 21 22
23 24
85, 1974 7
8
Devas M Stress fractures of the patella, in Devas M (ed). Stress Fractures Edinburgh, Churchill Livingstone, 1975, pp 130-137 Devas MB Stress fracture of the patella. J Bone Joint Surg 42B 71-74,
1960 9 Dickason JM, Fox JM Fracture of the patella due to overuse syndrome in a child A case report Am J Sports Med 10 248-249, 1982 10 Nordin M, Frankel VH Biomechanics of whole bones and bone tissue, in Frankel VJ, Nordin M (eds) Basic Biomechanics of the Skeletal System Philadelphis, Lea and Febiger, 1980, pp 15-60 11 Griswold AS Fractures of the patella Clin Orthop 4 44-56, 1954
25
26 27
Hanel DP, Burdge RE Consecutive indirect patellar fractures in an adolescent basketball player Am J Sports Med 9 327-329, 1981 Hayes WC, Boyle DJ, Velez A Functional adaptation in the trabecular architecture of the human patella Trans Orthop Res Soc 2. 114, 1977 Hayes WC, Snyder B Toward a quantitative formulation of Wolff’s law in trabecular bone Am Soc Mech Eng Appl Mech Dw 45 43-68, 1981 Huberti HH, Hayes WC Patellofemoral contact pressures The influence of Q-angle and tendofemoral contact J Bone Joint Surg 66A 715-724, 1984 lwaya T, Takatori Y Lateral longitudinal stress fracture of the patella. Report of 3 cases J Pediatr Orthop 5 73-75, 1985 Kaye JJ, Freiberger RH Fragmentation of the lower pole of the patella in
spastic lower extremities 97-100, Radiology1971 101 Mann M Fatigue fracture of the lower patellar pole in adolescents with cerebral movement disorders Z Orthop 122 167, 1984 Muller W Der militarische Abrissermudungsschaden. Deutsch Militararzt 8 283-286, 1943 McMaster PE Fractures of the patella Clin Orthop 4. 24-43, 1954 Nummi J Fracture of the patella A clinical study of 707 patellar fractures Ann Chir Gynaecol Suppl 179 1-85, 1971 Perry J, Antonelli D, Ford W Analysis of knee-joint forces during flexedknee stance J Bone Joint Surg 57A 961-967, 1975 Raux P, Townsend PR, Miegel R, et al Trabecular architecture of the human patella J Biomech 8 1-7, 1975 Reilly DT, Martens M Experimental analysis of the quadriceps muscle force and patello-femoral joint reaction force for various activities Acta Orthop Scand 43 126-137, 1972 Rosenthal RK, Levine DB Fragmentation of the distal pole of the patella in spastic cerebral palsy J Bone Joint Surg 59A 934-939, 1977 Sugiura Y, Ikuta Y, Muroh Y Stress fractures of the patella in athletes J Jpn Orthop Assoc 51 1421-1425, 1977 Townsend PR, Raux P, Rose RM, et al The distribution and anisotropy of the stiffness of cancellous bone in the human patella J Biomech 8 363367, 1975
28 Walter NE, Wolf MD Stress fractures 5 165-170, 1977
in
young athletes Am J Sports Med
Department of Orthopaedics, University of Washington, Seattle, Washington
Stress fracture of the patella is a rare injury. It was first described in the English literature by Devas in 1960,’ but previously noted by Muller in 1943.19 Devas described one longitudinal and one transverse stress fracture, as well as
transverse, minimally displaced, distal third patellar fracture
symptomatic bipartite patella suggesting stress fracture,
trochlear surface of the femur. The fracture was noncomminuted and the trabecular bone at the fracture site appeared sclerotic, both proximally and distally. The fracture was anatomically reduced and internal fixation using tension band wiring was performed (Fig. 2). The knee was placed in an immobilizer and the patient was allowed to bear weight as tolerated. On the 3rd postoperative day, he began active-assisted range of motion knee exercises. One week postoperatively he had no swelling and 90° of flexion. Two weeks postoperatively the patient was in a motor vehicle accident and braced himself for the impact using both arms and both legs. His knees did not contact the dashboard; however, he developed a moderate effusion in the previously injured knee. He was examined the following day and had no ecchymosis or lacerations. Radiographs taken at that time revealed widening of the fracture site. A second open reduction and internal fixation was performed using Kirschner wires, a tension band, and a malleolar screw. The patient began active-assisted knee motion exercises on the 2nd postoperative day. His subsequent course was unremarkable. By 6 weeks after surgery, the patient was walking without a limp. His knee moved from 0° to 100° of flexion and he had no extensor lag. Additional quadriceps rehabilitation and range of motion exercises were
one
(Fig. 1). At the time of surgery the patellar retinaculum was found to be intact and there was no evidence of contusion on the
in three male athletes aged 16, 23, and 28.’ Since that time, patellar stress fractures have been described in two groups of people, patients with cerebral palsy’, 17,18,25 and adolescent athletes.’ 9’z ls zs In the athletic group, patellar stress fractures have been reported in six boys aged 11 to 179 lz 16 and in one girl aged 10.16 These fractures included three longitudinal and four transverse stress fractures. The transverse fractures occurred during basketball, soccer, and high jump-
ing. The cases presented herein occurred during sports that different from those previously reported and may therefore have a distinct biomechanical etiology. One occurred during sailboarding and the other during belly dancing. Both activities require prolonged knee flexion and quadriceps contraction. The second case discussed here is the first report of patellar stress fracture in a woman and in a patient more than 30 years old. are
CASE REPORTS Case 1 A 23-year-old male ski instructor presented with acute right knee pain. He gave a history of several months of knee pain, suggestive of patellar tendinitis. This had not interfered with his skiing activities. As summer approached, he had begun sailboarding on a daily basis. While sailboarding on the day before presentation, he felt a sudden crack in his right knee. He was then unable to lift his right leg or bear weight on it. Physical examination of the patient’s right knee revealed a tense effusion and tenderness over the dorsum of the patella. The remainder of his knee examination was normal. Eighty milliliters of blood in which fat droplets were visible were aspirated from his right knee. Radiographs revealed a *
Address correspondence and repnnt requests 15, University of Washington, Seattle, WA 98195
to Carol C
prescribed. Six months later the patient was skiing without problems, but began to have symptoms related to the Kirschner wires’ subcutaneous position (Fig. 3). Approximately 1 year after the original fracture he underwent hardware removal. Six months later he worked as a ski instructor. Physical examination 20 months after the original injury revealed full range of motion, 0.5 inch of vastus medialis atrophy, and mild patellofemoral crepitus. Three and one-half years following fracture, the patient reported that his right knee was &dquo;excellent.&dquo; He denied swelling or pain, but noted mild residual weakness, such as inability to do a one-legged deep knee bend. He was skiing vigorously without problems.
Teitz, MD, GB761
762
Case 2 A
36-year-old woman presented with right knee pain. She reported a vague aching in her right knee for approximately 1 month before presentation. She stated that the cause of the pain was related to belly dancing, which she practiced approximately 2 to 3 hours per day and performed on weekends, 2 to 3 hours at a time. Three days before presentation, the patient was dancing when she felt a pop in her right knee and was unable to bear weight on that leg. She noted moderate swelling the following morning. Physical examination of the right knee revealed a small effusion and tenderness to palpation over the distal portion of the patella. There was no evidence of other tenderness or ligamentous laxity. Radiographic examination revealed a nondisplaced fracture of the distal third of the patella (Fig. 4). The patient’s knee was placed in a long leg cast for 4 weeks. At that time the patella was nontender and radiographs revealed fracture healing (Fig. 5). Quadriceps rehabilitation and range of motion exercises were initiated. When last seen 3 months after her injury, she was doing well and had no complaints. She had full range of motion, no patellofemoral crepitus, and had equal quadriceps strength in both legs.
DISCUSSION
Figure 2. A lateral radiograph taken after tension band wiring.
Because the patella lies within the quadriceps mechanism, the patellofemoral compression force is the resultant of the
Figure 3.
The patient’s right knee 1 year after internal fixation, before hardware removal.
secondary
763
patellar tendon force vector and the quadriceps force vector24 (Fig. 6). When the knee is flexed, a bending stress is applied to the patella, which is loaded in tension on its superficial surface and in compression on its deep surface. This recurrent bending stress produces in the patella a specific trabecular architecture that has been shown by quantitative steriologic analysis to correspond to the architecture predicted by finite element analysis.13,2’ Using a combination of contact microradiographs and finite element analysis of the patella, Hayes and associates&dquo; found that trabecular orientation conformed to principal stress directions, and trabecular density was related to the magnitude of the imposed stresses. Hayes and Snyder14 subsequently showed that the maximal principal stresses in the patella were tensile and that large tensile stresses (maximum, 8.5 MPa) were induced in the anterior patella by the combined action of the quadriceps and patellar tendons. This finding corresponded to the trabecular orientation in
4. A lateral radiograph of the second knee at initial presentation.
Figure
Figure 5. Four weeks after presentation, graph was taken.
patient’s right
this lateral radio-
Figure 6. Tensile forces are applied to the anterior surface of the patella through the quadnceps tendon (Fi) and the patellar tendon (F2). The posterior surface of the patella is loaded in compression. The patellofemoral compression force (R) is a function of F1 and F2 as well as of the angle of flexion of the knee.
764
the anterior patella, which is predominantly proximal to distal. 23 The subchondral region of the patella experienced maximal compressive stresses of 3 MPa. They concluded that the resultant stresses in the patella are similar to those in a short beam subjected to three-point bending with superimposed axial tension. The patellofemoral joint reaction force must be equal and opposite to the patellofemoral compression force and changes as a function of both the angle of flexion of the knee and the magnitude of quadriceps force, which increased 6% per degree of flexion. Reilly and Martens24 found that the patellofemoral joint reaction force was at least 0.5 times body weight during level walking, 3.3 times body weight going up and down stairs, and 7.6 times body weight during a deep knee bend. Huberti and Hayes15 estimated maximal patellofemoral contact forces of approximately 6.5 times body weight when the knee was flexed 90°. Perry et al.2z found that during stance, when the knee was in 30° of flexion, the quadriceps force required to stabilize the knee equaled 210% of body weight. They also found that this quadriceps force equaled 50% of the average maximum quadriceps strength. (In a 70-kg person, the quadriceps tension was measured at 1375 N when the knee was flexed to 30°.) These large quadriceps forces, markedly increased at 30° of flexion, enable us to proffer a mechanism whereby bending moments could produce a fatigue fracture of the patella initiated in tension, without requiring sudden violent muscle contraction or sudden knee flexion as in previously described cases. In addition, time-dependent damage may be caused by creep in addition to or rather than fatigue. The total damage in a living, self-healing structure at any moment in time is the summation of damage caused by disease and aging plus the damage caused by mechanical stress, reduced by the damage repaired by healing processes. In human bone, the damage caused by mechanical stress is the summation of damage caused by cyclically applied loading (fatigue damage) and damage caused by steady application of load (creep damage). Carter and Hayes’ conducted cycle-dependent fatigue tests on human bone at physiologic temperature and loading rates. The tests showed that bone fatigue damage is diffuse in the tissue and causes a progressive decrease in strength and stiffness. Carter and Caler;J also conducted time-dependent creep fracture tests on human bone. The steady application of load resulted in the progressive accumulation of cracks within the material, with such accumulation dependent on both magnitude and duration of loading. Patellar fractures caused by &dquo;indirect force&dquo; have been described previously, 11 20 although their incidence in a large study of 707 patellar fractures was 1.5%21 and there was none in another study of 422 patellar fractures.’ In these reports, patellar fractures not caused by a direct blow were thought to be caused by sudden, violent quadriceps contraction or by high tensile forces created by the quadriceps muscle and patellar ligament. In all cases, the fractures were associated with extensive rupture of the patellar retinaculum.
Stress fractures generally occur because of high repetitions of loads that are below the yield strength of a given material.’ In living bone, fatigue fracture is not merely a function of cyclical loading but also depends on the frequency of loading.l° Fatigue fracture results when the frequency of loading precludes the remodeling necessary to prevent failure. Hence, it is the result of the amount and duration of load in concert with the degree of reparative ability. Muscle strength can also affect the likelihood of stress fracture, either by shielding the bone from applied stresses’ or by loading the bone as a function of increasing muscle contraction. The latter applies to the patella, which is loaded as a function of quadriceps force. Walter and Wolf&dquo; felt that athletes could repetitively load bone close to its yield strength because of superior muscle tone. In the cases previously described by Hanel and Burdge, 12 Sugiura et al.,26 and Dickason and Fox,9 the transverse patellar fractures occurred when the knee was in a flexed position and a sudden forceful contraction occurred in the process of jumping or kicking. Whether previous submaximal tensile loads had produced a weakened area in the patella is unknown and we have no history of previous complaints in these patients. The transverse fracture previously described by Devas’ occurred in a man who ran frequently and who theoretically required no explosive burst of quadriceps power. Patellar stress fractures also have been described in cerebral palsy patients with spastic lower extremities, many of whom had knee flexion contractures. 2,17 18 25 In both cases presented here, the patients’ activities required prolonged isometric quadriceps contraction with the knee in approximately 30° to 45° of flexion. All of the cases mentioned here may represent stress fractures with slightly different characteristics. In the kickers and jumpers the loads may have been of high magnitude, thereby requiring few cycles to fatigue. This theory is corroborated by the soft tissue damage noted in the retinaculum in these previously described patients. In cerebral palsy patients, the loads may be of varying magnitude and occur tonically or clonically. In our patients with no soft tissue damage, the positions and quadriceps contractions required by their activities may have produced a fracture because of creep or because of fatigue from relatively low loads cycled often enough to prevent repair.
CONCLUSIONS Two
presented of transverse patellar stress fracpatients whose activities required prolonged isometric quadriceps contraction and relatively constant knee cases are
tures in
flexion. We theorize that these stress fractures occurred because of repeated bending moments (possibly in association with creep) that initiated a fracture in tension on the superficial surface of the patella. Stress fractures on the tensile surface in other bones (e.g., femoral neck) are more likely to proceed to complete fracture, or heal more poorly than those on the compressive surface.l° Patients whose
765
activities require frequent knee flexion and quadriceps use, either tonically or in sudden bursts, are at risk for these fractures. Such patients presenting with knee pain and superficial patellar tenderness should undergo radiographic examination and possibly bone scan to identify early stress fractures of the patella before they become complete fractures.
REFERENCES
12
13 14
15
16 17 18
1
2 3 4
5 6
Brostrom A Fracture of the patella A study of 422 patellar fractures Acta Orthop Scand (Suppl) 143 1-80, 1972 Cahuzac M, Nichil J, Olle R, et al Les fractures de fatigue de la rotule chez l’infirme moteur de’origine cerebrale Rev Chir Orthop 79 87, 1965 Carter DR, Caler WE A cumulative damage model for bone fracture J Orthop Res 3 84-90, 1985 Carter DR, Hayes WC Compact bone fatigue damage A microscopic examination Clin Orthop 127 265-274, 1977 Chamay A Mechanical and morphological aspects of experimental overload and fatigue in bone J Biomech 3 263-270, 1970 Clement DB Tibial stress syndrome in athletes Am J Sports Med 2 81-
19
20 21 22
23 24
85, 1974 7
8
Devas M Stress fractures of the patella, in Devas M (ed). Stress Fractures Edinburgh, Churchill Livingstone, 1975, pp 130-137 Devas MB Stress fracture of the patella. J Bone Joint Surg 42B 71-74,
1960 9 Dickason JM, Fox JM Fracture of the patella due to overuse syndrome in a child A case report Am J Sports Med 10 248-249, 1982 10 Nordin M, Frankel VH Biomechanics of whole bones and bone tissue, in Frankel VJ, Nordin M (eds) Basic Biomechanics of the Skeletal System Philadelphis, Lea and Febiger, 1980, pp 15-60 11 Griswold AS Fractures of the patella Clin Orthop 4 44-56, 1954
25
26 27
Hanel DP, Burdge RE Consecutive indirect patellar fractures in an adolescent basketball player Am J Sports Med 9 327-329, 1981 Hayes WC, Boyle DJ, Velez A Functional adaptation in the trabecular architecture of the human patella Trans Orthop Res Soc 2. 114, 1977 Hayes WC, Snyder B Toward a quantitative formulation of Wolff’s law in trabecular bone Am Soc Mech Eng Appl Mech Dw 45 43-68, 1981 Huberti HH, Hayes WC Patellofemoral contact pressures The influence of Q-angle and tendofemoral contact J Bone Joint Surg 66A 715-724, 1984 lwaya T, Takatori Y Lateral longitudinal stress fracture of the patella. Report of 3 cases J Pediatr Orthop 5 73-75, 1985 Kaye JJ, Freiberger RH Fragmentation of the lower pole of the patella in
spastic lower extremities 97-100, Radiology1971 101 Mann M Fatigue fracture of the lower patellar pole in adolescents with cerebral movement disorders Z Orthop 122 167, 1984 Muller W Der militarische Abrissermudungsschaden. Deutsch Militararzt 8 283-286, 1943 McMaster PE Fractures of the patella Clin Orthop 4. 24-43, 1954 Nummi J Fracture of the patella A clinical study of 707 patellar fractures Ann Chir Gynaecol Suppl 179 1-85, 1971 Perry J, Antonelli D, Ford W Analysis of knee-joint forces during flexedknee stance J Bone Joint Surg 57A 961-967, 1975 Raux P, Townsend PR, Miegel R, et al Trabecular architecture of the human patella J Biomech 8 1-7, 1975 Reilly DT, Martens M Experimental analysis of the quadriceps muscle force and patello-femoral joint reaction force for various activities Acta Orthop Scand 43 126-137, 1972 Rosenthal RK, Levine DB Fragmentation of the distal pole of the patella in spastic cerebral palsy J Bone Joint Surg 59A 934-939, 1977 Sugiura Y, Ikuta Y, Muroh Y Stress fractures of the patella in athletes J Jpn Orthop Assoc 51 1421-1425, 1977 Townsend PR, Raux P, Rose RM, et al The distribution and anisotropy of the stiffness of cancellous bone in the human patella J Biomech 8 363367, 1975
28 Walter NE, Wolf MD Stress fractures 5 165-170, 1977
in
young athletes Am J Sports Med
