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© 1998 The Journal of Bone and Joint Surgery, Inc. The Load Applied to the Foot in a Patellar Ligament-Bearing Cast* , BALTIMORE, MARYLANDInvestigation performed at the Maryland Center for Limb Lengthening and Reconstruction, Baltimore
The purpose of this study was to determine whether a patellar ligament-bearing cast reduces the load applied to a foot in a cast. In a study of ten people who had no history of gait abnormalities, disease involving the motor system, or deformities of the lower extremities, we compared the load applied to the plantar aspect of a foot in a cast (as detected with F-Scan computer-monitored pedobarographic sensors) with the total load that an extremity in a cast receives relative to the ground (as detected with force-plates). Six trials were completed three times by each person. The trials consisted of walking (1) while wearing regular shoes; (2) with a patellar ligament-bearing cast on one leg; (3) with a patellar ligament-bearing cast and an overlying soft knee brace, locked in full extension, on the leg; (4) with only a below-the-knee cast on the leg; (5) with a below-the-knee cast and an overlying knee brace, locked in full extension, on the leg; and (6) with only a knee brace, locked in full extension, on the leg. The loads at peak heel-strike for all three trials were averaged and normalized to body weight. The load on the plantar aspect of the foot, as compared with the total load, was reduced a mean of 11 percent when the patellar ligament-bearing cast was worn alone, and it was reduced a mean of 26 percent when the patellar ligament-bearing cast was used with an overlying knee brace locked in full extension. This difference was significant (p = 0.007). With the numbers available, we could not detect a significant difference between the reduction in load when a patellar ligament-bearing cast was worn alone compared with that when a below-the-knee cast was worn alone or between the reduction when a below-the-knee cast was worn alone compared with that when a below-the-knee cast was used with a knee brace (p = 0.3). In conclusion, we could not demonstrate a significant reduction in the load on the foot when a patellar ligament-bearing cast was used in a traditional fashion; however, a significant (p = 0.007) reduction in load was found when a knee brace locked in full extension was worn in addition to the patellar ligament-bearing cast.
Patellar ligament-bearing casts were first described by Sarmiento in 1967 for the treatment of tibial fractures16. He developed the patellar ligament-bearing cast by adapting the design of a prosthesis developed at the University of California at Berkeley for patients who had had a below-the-knee amputation. Numerous clinical investigations have supported the use of patellar ligament-bearing casts for low-energy fractures with reported low rates of nonunion and of complications1-3,12,13,17,19. The indications for the use of patellar ligament-bearing casts and braces have since expanded to include neuropathic conditions of the foot8,10, osteoarthrosis of the ankle and subtalar joints,15, fracture of the calcaneus15, and avascular necrosis of the talus15; the casts have also been used after arthrodesis of the ankle15. The firm molding of the cast over the medial tibial flare, patellar ligament, and popliteal space theoretically absorbs the load applied to an extremity in a cast, so that the load bypasses the fracture site and thus, in effect, suspends the fractured bones16. Sarmiento, however, initially questioned the role of such molding in the distribution of weight-bearing pressure16. He later theorized that weight-bearing resulted in shortening of the tibia at the fracture site with expansion of the soft tissues against the rigid walls of the cast, which stabilized the fracture and decreased the load at the fracture site17,18. Regardless of its mechanism, the desired outcome was early, limited functional loading at the fracture site without loss of rotational or axial stability. Dehne postulated that this allows for regulated mechanical stress at the fracture site, which is thought to be a primary factor in the process of fracture repair5,6. Several biomechanical studies have been performed to quantify the unloading effect of such casts. Mimura found a 30 percent reduction in load with the use of patellar ligament-bearing casts11. Other authors have documented little, if any, decrease in load transmission with the use of these casts compared with the decrease provided by below-the-knee casts15,21. Previous investigators have measured either the force vector at the interface of the cast and the floor with force-plates or the load on the plantar surface of the foot inside the cast with pressure-sensitive sensors9. However, both systems must be used to measure the net force accurately and thus to determine conclusively whether a patellar ligament-bearing cast eliminates a major percentage of the vertical load transmitted to an extremity.
Five men and five women who were nineteen to thirty-five years old volunteered for this study. They had no history of gait abnormalities, disease involving the motor system, or deformities of the lower extremities. This study was approved by the institutional review board, and informed consent was obtained. Each person had F-Scan sensors (Tekscan, Boston, Massachusetts) placed on the plantar surfaces of both feet. F-Scan sensors are thin (0.178-millimeter), flexible Mylar (polyester) sheets incorporating 960 sensors that are evenly distributed at five-millimeter intervals in a grid-like pattern22. Individual sensors consist of intersecting rows and columns of conductors separated by a material that varies its electrical resistance with applied load. Sequential scanning of each row and column at 100 times per second allows the computer system to record and graph the amount of force at each point of intersection during the entire gait cycle. Individual sensing cells have a pressure-recording range of eight to 124 pounds per square inch (55.2 to 855.0 kilopascals). The sheets may be trimmed to accommodate shoe sizes ranging from a man's size fourteen to a child's size two, and they are thin enough to be inserted as an insole into footwear without altering gait patterns. Only one set of sensors was used for each person so that the inherent variability between sensors, as demonstrated by Rose et al.14, would be eliminated. The sensors were trimmed to cover the entire plantar surface of the foot, and they were held in place with adhesive tape and an overlying stockinette. The sensors were attached to a converter unit that was secured to the leg of each volunteer with a Velcro cuff (Fig. 1). A coaxial cable connected the converter to a 486 IBM-compatible computer, which provided graphs of force versus time (Fig. 2) for the loads at the cast-foot interface at heel-strike (T1) and toe-off (T3) with use of Tekscan systems software. Data were sampled at 300 hertz during the entire gait cycle. This provided measurements of the vertical ground-reaction force vector applied to the foot inside the cast.
Simultaneous recordings of the ground-reaction force vector at the cast-floor interface were made with two AMTI OR6-5 force-plates (Advanced Medical Technology, Watertown, Massachusetts). The two force-plates, which have a maximum capacity of approximately 900 kilograms for load, were arranged end-to-end to allow ample space for walking at a normal gait while measurements were obtained. These plates are specifically designed to measure ground-reaction forces along orthogonal axes x, y, and z through the use of strain-gauges mounted on four separate components within the platforms. However, only the values for the vertical ground-reaction force vector (axis z) from the first force-plate were used in our study because they correspond to simultaneously measured heel-strike values. Data were gathered from the gauges incorporated into the plates at a frequency of 1000 hertz. The force-plates were recessed into an 18.3-meter walking platform so that no alteration in gait pattern was necessary to cross the plates. An amplifier (AMTI MCA series; Advanced Medical Technology) connected the force-plates to the 486 IBM-compatible computer. Force-plate data were analyzed with Ariel Systems software (Ariel Life Systems, San Diego, California). This generated graphs of force versus time for the loads at the cast-floor interface at heel-strike (F1) and toe-off (F3) during the stance phase of the gait cycle (Fig. 2). Six sets of trials were performed three times each. The values that were obtained were normalized to the body weight of each volunteer, and the values for the three trials in each set were averaged for analysis. Each volunteer was asked to walk at his or her preferred rate over the force-plates for each trial. The first set of trials, which consisted of walking while wearing regular shoes, was performed after calibration of the sensors to the body weight of the volunteer. For the second set of trials, each person wore a fiberglass patellar ligament-bearing cast that was well molded in the medial tibial plateau and infrapatellar regions, as described by Sarmiento17. All of the casts were applied and trimmed by the same experienced cast technician. A 4.0-by-0.5-centimeter window on the lateral aspect of the cast allowed a sensor to be connected to the converter box. Once sensors had been applied and calibrated, they were not disconnected until all of the recordings were obtained. Patellar ligament-bearing casts work best when the knee is full extension, which maximizes contact between the cast and the patellar ligament and maximizes load-bearing by the medial tibial plateau and the patellar ligament. Therefore, for the third set of trials, the leg in the patellar ligament-bearing cast was kept locked in full extension with an externally applied Bledsoe brace (Medical Technology, Grand Prairie, Texas). For the fourth set of trials, the cast was modified into a standard below-the-knee cast with its proximal extension just distal to the tibial tubercle. This was followed by the fifth set of trials, which measured the load on the foot of the volunteers while they wore a below-the-knee cast with an overlying Bledsoe brace locked in full extension. Finally, the cast was removed and the sixth set of trials was performed with the volunteers wearing only a Bledsoe brace locked in full extension.
Statistical Analysis
We measured the percent load reduction (F1 minus T1) at peak heel-strike because this phase corresponds to the weight-acceptance phase of gait and it is representative of maximum loading of the extremity in a cast. Toe-off was not used for analysis. As mentioned, the ten volunteers performed each of the six trials three times. Previous authors have studied changes in total pressure applied by the involved extremity at the cast-floor interface9,15. However, because each person does not walk precisely the same way every time (that is, at the same speed or with the same movements of the upper and lower extremities), we noted that there was a considerable difference in the total load applied to the involved extremity by each person in the different sets. This was shown by differences in the peak loads at the cast-floor interface during the loading response. Therefore, only the differences between the F1 and T1 values were used for each analysis. In this way, we were able to isolate load measurements on the foot in the cast as a function of the cast design. The mean decrease in load transmission (F1 minus T1) was 26 percent (range, 7 to 53 percent) when the patellar ligament-bearing cast was worn with an overlying Bledsoe brace locked in full extension, whereas the mean decrease was only 11 percent (range, 6 to 25 percent) when the patellar ligament-bearing cast was worn without the brace (Table I). Similarly, when the below-the-knee cast was worn alone, load transmission decreased a mean of 13 percent (range, 1 to 37 percent), whereas the below-the-knee cast with a brace provided a mean decrease of 7 percent (range, 1 to 17 percent). Footwear alone caused a mean reduction in load transmission of 5 percent (range, 0 to 10 percent). There was a mean reduction in load transmission of 4 percent (range, 1 to 8 percent) when a Bledsoe brace locked in full extension was worn alone. The difference between the reduction in load provided by the patellar ligament-bearing cast with an overlying Bledsoe brace and the reductions measured during the other trials was significant (p = 0.007). No significant difference was demonstrated between the reductions measured during any of the other trials (p = 0.03) (Fig. 3).
Early functional loading of stable fractures of the lower extremity has gained widespread acceptance since it was described by Dehne et al. in 19614. Patellar ligament-bearing casts have been said to reduce ultimate loads at the fracture site, although the importance of the amount of reduction has been disputed16-18. In our study, we did not find, with the numbers available, that the patellar ligament-bearing cast alone significantly reduced loading at the cast-foot interface relative to that at the cast-floor interface. The peak load at the cast-foot interface at heel-strike is representative of the loading of the lower extremity that would be seen at the site of a tibial fracture. Use of the locked extension brace maximized load-bearing by the medial tibial plateau and the inferior patellar pole. Normal walking, with its typical 20 degrees of knee flexion during the early stance phase20, theoretically negated this distribution of load to the proximal aspect of the tibia and the patellar ligament; this accounted for the difference between the results with the patellar ligament-bearing cast alone and those with the patellar ligament-bearing cast with an extension brace. In light of our findings, clinical applications of patellar ligament-bearing casts for reduction of load must be questioned. A decrease in the transmission of load to the foot was found only when a patellar ligament-bearing cast (which allows for early functional weight-bearing) and a knee brace locked in extension were worn simultaneously. The use of patellar ligament-bearing cast with a knee brace locked in extension minimizes the potential benefits of maintaining a greater range of knee motion and the documented return of function after an injury7. We did not directly address the use of patellar ligament-bearing casts for the treatment of disorders of the forefoot and midfoot in our study because we were concerned primarily with loads transmitted to the hindfoot and the tibial shaft, as represented by the differences between the F1 and T1 values. Saltzman et al., using F-Scan sensors in a study of Charcot changes in the midfoot, were not able to detect a significant difference in the mean peak loads under the midfoot when the patients wore a patellar ligament-bearing brace15. Thus, they did not recommend the use of a patellar ligament-bearing brace. The same study demonstrated a 37 percent decrease in peak load transmission to the hindfoot. This differs from the 11 percent decrease that was found in our investigation. However, the free-motion ankle hinge that was used in their study allowed for the dispersion of generated force through eccentric plantar flexion of the ankle; this is not possible with the patellar ligament-bearing cast. A more important aspect of our study was the fact that the values for load transmission were not recorded at the cast-floor interface only. Through the simultaneous use of force-plates and F-Scan sensors, we were able to obtain a ratio between the total load applied to the affected lower extremity (the cast-floor interface) and that applied to the foot in the cast (the cast-foot interface). We believe this to be a more accurate representation of the function of the cast design, independent of changes in gait patterns. Changes in gait patterns could decrease the load applied to the extremity in the cast, thereby falsely decreasing load measurements within the cast. The limited durability of the F-Scan insole sensors may be a confounding variable in our investigation. According to Rose et al., the sensors may be used for approximately thirty gait cycles; after this, the accuracy of the pressure recordings begins to decrease steadily secondary to wear on the individual pressure-sensing cells14. We therefore limited the recording during each trial to approximately three or four gait cycles. We also completed the trials with the patellar ligament-bearing cast and the patellar ligament-bearing cast with a brace early, when the F-Scan sensors were deemed most accurate. Our decision not to exchange the F-Scan sensors was based on the work of Rose et al., which demonstrated considerable variability in pressure recordings between different sensors14. Our results demonstrated a reduction in the pressure recordings when the patellar ligament-bearing cast was worn with a Bledsoe brace but an increase in the pressures recorded in subsequent trials with the patellar ligament-bearing cast alone, the below-the-knee cast with a brace, and the below-the-knee cast without a brace. If limited durability (decreased sensitivity of the cells) were indeed a confounding factor, the later trials would have shown a reduction in load, not the increased values that we observed. If anything, the decreasing sensitivity of the F-Scan sensors falsely narrowed the difference between the patellar ligament-bearing cast with a Bledsoe brace and the other braces that we tested. Maintenance of rotational control of the proximal aspect of the tibia is another proposed benefit of a patellar ligament-bearing casts. This issue was not addressed in the present study, and it remains a theoretical advantage that needs further investigation. However, many authors have reported excellent clinical results after the treatment of low-energy fractures of the tibial shaft with a patellar ligament-bearing cast1,12,23. The results of our study do not refute those reports, and we are not proposing a new method of conservative management. However, we did not find that a patellar ligament-bearing cast used without maintenance of full extension of the knee afforded a greater reduction of load to the lower extremity than did a traditional below-the-knee cast. We therefore agree with Svend-Hansen et al. that the choice of treatment must be based on other factors21.
*No benefits in any form have been received or will be received from a commercial party related directly or indirectly to the subject of this article. No funds were received in support of this study.
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