Comparing the Efficacy of Kneeling Stress Radiographs and Weighted Gravity Stress Radiographs to Assess Posterior Cruciate Ligament Insufficiency

Abstract

Background:

Kneeling posterior cruciate ligament (PCL) stress radiographs are commonly used to evaluate PCL laxity. Patients, however, report significant pain, and the method’s reproducibility may be challenged due to its dependence on patient body weight distribution to produce posterior tibial displacement. Weighted gravity stress radiography may offer better reproducibility and comfort than the kneeling technique, but its efficacy has not been studied.

Hypothesis:

Weighted gravity PCL stress radiographs will be more comfortable and produce similar measurements of side-to-side difference in posterior tibial displacement when compared with the kneeling technique.

Study Design:

Cohort study (diagnosis); Level of evidence, 3.

Methods:

A total of 40 patients with nonoperatively or >6 months postoperatively treated PCL injuries (isolated or multiligamentous) underwent bilateral stress radiographs. Weighted gravity and kneeling stress radiographs were acquired, in random order, for each patient, as well as side-to-side difference in posterior tibial displacement between each knee, patient-reported visual analog scale knee pain (100 mm), time to acquire the images, and patient preference for technique. Paired t tests were used to compare the side-to-side difference, pain score, and time to complete the radiographs.

Results:

There was no difference between the 2 radiographic methods in the mean side-to-side difference (gravity: 6.45 ± 4.61 mm, kneeling: 6.82 ± 4.60 mm; P = .72), time required to acquire radiographs (kneeling: 307.3 ± 140.5 seconds, gravity: 318.7 ± 151.1 seconds; P = .073), or number of radiographs taken to obtain acceptable images (kneeling: 3.6 ± 1.6, gravity: 3.7 ± 1.7; P = .73). Patients reported significantly less knee pain during the weighted gravity views (kneeling: 31.8 ± 26.6, gravity: 4.0 ± 12.0; P < .0001). Of the patients, 88% preferred the weighted gravity method.

Conclusion:

Weighted gravity stress radiographs produce similar side-to-side differences in posterior tibial translation compared with the kneeling stress technique, but do not rely on patient weightbearing and provide significantly better patient comfort. Clinicians should therefore consider the use of weighted gravity stress radiographs in clinical practice to minimize the pain associated with stress radiography while allowing for accurate decision making.

Keywords

knee posterior cruciate ligament stress radiography general sports trauma

In cases of suspected posterior cruciate ligament (PCL) injuries, stress radiographs are acquired to assess the amount of posterior knee laxity as quantified by the extent of posterior tibial displacement (PTD) when a posteriorly directed force is applied to the proximal tibia. Measurement of the side-to-side difference (SSD) in PTD between the healthy and PCL-injured knee can aid in determining the extent of ligamentous and capsular injury.⁸ Stress radiographs may be particularly helpful in determining the posterior laxity associated with acute partial tears or chronic tears where the lax PCL has healed and a magnetic resonance imaging scan demonstrates the PCL is in continuity. Stress views may also be helpful in knees with bicruciate injuries to determine the contribution of the PCL injury to the overall amount of anteroposterior translation. There are multiple stress radiography techniques currently used to measure PCL insufficiency. Among these, the sensitivity, specificity, positive predictive value, and negative predictive value for PCL injuries are consistently high.^{8,10,11,14,15,21} These techniques, however, each carry limitations that have prevented the development of a clear consensus on the best technique to use.¹⁰

In the systematic review by James et al,¹⁰ the authors suggested that an ideal stress radiograph would be inexpensive, efficient, reproducible, accurate, and examiner independent. In addition, an ideal technique should minimize patient pain and discomfort during the measurement, as this pain can cause the patient to contract stabilizing muscles around the knee, which may reduce the amount of posterior tibial translation.¹¹ Furthermore, in techniques that depend on patients to bear weight through the knee, pain may limit the amount of pressure they put through the knee, reducing PTD. This potential reduction in PTD from pain-related muscular activation or weight shifting may therefore inappropriately alter management decisions that are based on the results of the PCL stress radiographs.^11,18

In a comparison study by Jung et al,¹¹ the Telos device, hamstring contraction,³ kneeling view,^8,13,14 gravity view,^21,22 and axial view¹⁵ stress radiography techniques were assessed based on PTD, time per radiograph, and patient pain. The authors concluded that the Telos method and kneeling view produced the largest PTD values.¹¹ The Telos method, however, took the most time to complete, caused the most pain, and was the most expensive.¹¹

The kneeling stress radiograph¹⁴ has been suggested as a reliable alternative to the Telos device given its efficiency, cost-effectiveness, and reported ability to reliably produce large values for PTD and SSD.^6-8,11,14 This technique, however, causes significant pain^1,7,10,11 and may have issues with reproducibility as it is dependent on patient body weight and balance to distribute the appropriate load while maintaining a potentially painful position.¹⁴ Additionally, it has been suggested that pain associated with this technique results in muscle guarding that may limit the amount of PTD produced.^11,22

The gravity stress view uses the weight of the shank to produce PTD.^21,22 Compared with the Telos and kneeling views, the gravity view was faster and was found to be more comfortable, but produced less PTD.¹¹ The current study introduces the use of a weighted gravity stress view that was developed to increase the production of PTD in a standard, cost-effective, and reproducible manner, while maintaining patient comfort and efficiency of measurement.

The specific aims of this study were to determine (1) if the weighted gravity stress view produces SSD values that are comparable with those of the kneeling stress view and (2) if the weighted gravity stress technique offers a more comfortable option for patients compared with the kneeling view. The hypotheses were that (1) the weighted gravity stress view would produce similar SSD values and (2) it would be more comfortable to the patient, when compared with the kneeling stress view.

Methods

In total, 40 patients aged 18 to 70 years with nonoperatively or more than 6 months postoperatively treated PCL injuries (isolated or multiligamentous) were recruited from 2 academic level 1 trauma centers. Patients were excluded if they were pregnant, could not provide consent, had recent fractures or surgery or open wounds involving the proximal tibia/knee/femur, were less than 6 months after PCL or posterolateral corner repair/reconstruction, or had acute or chronic bilateral PCL or posterolateral corner injuries.

Approval from the institutional review board was obtained from the senior author’s (J.A.G.) institution and a data-sharing agreement was put in place. Patients were enrolled between April 2017 and February 2018. All patients consented to have bilateral lateral PCL stress radiographs taken during a single session using both the kneeling stress view and the weighted gravity stress view methods for each knee. The order of acquisition of the radiographic techniques (kneeling vs weighted gravity) was randomized. The radiographs were obtained by experienced radiology technicians. The radiology technicians were instructed to capture radiographs that met quality expectations for typical clinical use, and the number of attempts needed to acquire satisfactory images was recorded. The weighted gravity stress view (Figure 1) was conducted with the patient lying supine with the hip and knee of interest being placed at 90° of flexion with the heel supported. Subsequently, a 20-lb (89-N) weight was placed on the anterior midshaft of the tibia just distal to the tibial tubercle. For the kneeling stress view (Figure 2), the patient was positioned to kneel on a padded stool with the knees at 90° of flexion and instructed to distribute his or her weight evenly through both knees. Contact of the tibia with the padding was positioned just distal to the tibial tubercle. The stool had a separate pad for each knee, and beneath each pad was a calibrated scale that recorded the body weight transmitted through each individual knee. The scale output was not visible to the patient. A handrail was placed in front of patients for stability, but they were encouraged not to use it to support their body weight.

The weight distributed through each leg was recorded for the kneeling technique. The time required for the completion of each technique was recorded from the beginning of patient positioning to the completion of a clinically acceptable series of radiographs. Patients completed a 100-mm visual analog scale (VAS) directly after each of the respective techniques to report the level of knee pain associated with each method. Additionally, patients subjectively reported which technique they preferred.

The stress radiographs were deidentified, and measurements of PTD and SSD were performed in picture archiving and communication system software. To measure the PTD, 2 parallel lines perpendicular to the tibial plateau were drawn: 1 in contact with the midpoint of the most posterior aspect of the femoral condyles and 1 in contact with the midpoint of the most posterior aspect of the tibial condyles (Figure 3).^11,17 A line perpendicular to both of these was drawn to measure the distance between them. When the tibial line was posterior to the femoral line, translation was labeled as positive. The difference between measurements of PTD between the PCL-intact and PCL-injured knees was calculated and recorded as the SSD. A positive value represents increased posterior translation of the PCL-injured tibia. Measurements were performed by 2 sports medicine fellowship-trained orthopaedic surgeons (J.A.G., R.M.) with experience in the management of PCL injuries and a medical student (intrarater reliability: 0.79-0.87; interrater reliability: 0.73-0.74).

Data Analysis

Means, standard deviations, and boxplots were calculated for all continuous variables. A sample size of 31 patients was needed to detect a 2-mm difference (α = .05; power, 80%; SD, 2.8 mm).¹¹ Paired Student t tests (α < .05) were used to compare the SSD in PTD, VAS pain, distribution of weight between PCL intact and PCL-injured knees (kneeling method), number of radiographs taken, and time to complete the series for the kneeling and weighted gravity stress views.

Results

Forty participants (9 women, 31 men; mean age, 34.5 ± 12.8 years) were recruited. The right knee was injured in 35% of patients. There were 9 patients with isolated PCL injuries and 31 patients with multiligamentous injuries (Table 1). Seventeen patients (42.5%) were more than 6 months after PCL reconstruction while the remaining 23 patients had not undergone PCL reconstruction. PCL and multiligamentous injuries were all reconstructed with allograft (PCL with single-bundle Achilles tendon) except for 1 quadriceps tendon autograft PCL. There was no significant difference in time to perform the series of radiographs, number of radiographs needed per series, or SSD in PTD between kneeling and weighted gravity methods (Table 2). This trend remained when patients were stratified into PCL-deficient or PCL-reconstructed groups.

Table 1

Distribution of Knee Ligament Injury Diagnoses (N = 40)^a

	No.
Isolated PCL	9
PCL/MCL	9
PCL/PLC	4
PCL/LCL/MCL	1
PCL/ACL	1
PCL/ACL/LCL	2
PCL/ACL/PLC	8
PCL/ACL/MCL	6

ACL, anterior cruciate ligament; LCL, lateral collateral ligament; MCL, medial collateral ligament; PCL, posterior cruciate ligament; PLC, posterolateral corner.

Table 2

Outcomes Comparing the Kneeling PCL Stress View to the Weighted Gravity Stress View for Evaluation of PCL Insufficiency^a

		Mean ± SD		Mean Difference
	No.	Kneeling	Weighted Gravity	(kneel-weighted)	95% CI (difference)	P Value
Overall SSD in PTD, mm	40	6.82 ± 4.60	6.45 ± 4.61	0.37	−1.68 to 2.42	.72
PCL deficient: SSD in PTD, mm	23	7.80 ± 4.75	7.85 ± 5.14	−0.05	−1.67 to 1.76	.96
Post-PCLR: SSD in PTD, mm	17	5.48 ± 4.00	4.55 ± 2.99	0.93	−1.01 to 2.87	.33
Isolated PCL deficient: SSD in PTD, mm	9	8.12 ± 4.00	6.84 ± 3.23	1.28	−2.19 to 4.74	.42
Isolated PCLR: SSD in PTD, mm	0	—	—	—	—	—
Multiligament deficient: SSD in PTD, mm	14	7.60 ± 5.46	8.49 ± 6.09	−0.89	−2.91 to 1.12	.36
Multiligament reconstructed SSD in PTD, mm	17	5.48 ± 4.00	4.55 ± 3.77	0.93	−1.01 to 2.87	.33
Time to obtain radiographs, sec	40	307.3 ± 140.5	318.7 ± 151.1	−11.43	−77.6 to 54.8	.073
No. of radiographs needed for acceptable image	40	3.6 ± 1.6	3.7 ± 1.7	−0.13	−0.87 to 0.61	.73
Knee pain, VAS mm	40	31.8 ± 26.6	4 ± 12	27.77	18.29 to 37.25	<.0001

Boldface indicates statistical significance. Dashes indicate no comparative statistics available. PCL, posterior cruciate ligament; PCLR, PCL reconstruction; PTD, posterior tibial displacement; SSD, side-to-side difference; VAS, visual analog scale.

During the kneeling stress radiographs, patients applied more force through the uninjured knee compared with the PCL-injured knee (Table 3); however, this difference was not statistically significant (P = .087). When the difference in the force applied between the PCL-injured and uninjured knees was stratified by PCL-deficient or PCL-reconstructed patients, the mean difference in the force applied through each knee was similar. The variability, however, was much higher in the PCL-reconstructed group. For the PCL-deficient group, the difference between the 2 knees ranged from −68.6 N to 164.8 N while the range for the PCL-reconstructed group was −307.7 N to 764.4 N (a negative value denotes more force placed through the PCL-injured knee).

Table 3

Force (N) Placed Through Each Knee During the Kneeling Stress Views^a

	Total (N = 40)	PCL Deficient Mean (n = 23)	PCL Deficient Min, Max (n = 23)	PCLR Mean (n = 17)	PCLR Min, Max (n = 17)
PCL-injured leg	210.7 ± 110.7	233.0 ± 96.2	71.3-374.2	181.7 ± 124.9	9.8-499.8
Normal leg	255.8 ± 118.6	277.1 ± 80.3	107.8-400.9	228.7 ± 154.1	98.0-774.2
Difference	45.4 ± 155.9	42.1 ± 54.9		47.0 ± 234.7

Data are reported as mean ± SD or range. PCL, posterior cruciate ligament; PCLR, PCL reconstruction.

On the VAS pain scale, there was significantly less pain reported for the weighted gravity stress method compared with the kneeling stress method (kneeling: 31.8 ± 26.6 mm, weighted gravity: 4.0 ± 12 mm; P < .0001). Of the 40 patients, 35 (88%) reported a subjective preference for the weighted gravity stress view technique, although 3 of the remaining 5 patients did not have a preference.

Discussion

The current study demonstrates the ability of a low-cost weighted gravity stress radiograph to be used interchangeably with the more common kneeling stress views as a diagnostic tool for the assessment of PCL injury. Several stress radiography techniques have been described to assess for suspected PCL injuries, with kneeling stress radiographs commonly being endorsed as a preferred method. The kneeling stress technique has been praised for its simplicity, large displacing forces, and its ability to consistently produce large values for PTD and SSD in PTD.^6-8,11,14 It has been criticized, however, for causing significant patient discomfort as well as for its dependence on patient body weight distribution, balance, and effort to distribute the displacing forces.^1,8,10,11,14

The results of this study indicate that the weighted gravity method is equally as effective as the kneeling method in producing sufficient SSD in PTD to characterize PCL insufficiency. These results are comparable with those of Jung et al,¹¹ who found that, while the kneeling stress technique produces larger values of PTD, there was no statistically significant difference between the absolute PTD or SSD in PTD produced by the kneeling stress technique when compared with other methods of stress radiography.

The results of the current study indicate that the kneeling stress technique may result in differences in the amount of body weight placed through the uninjured and PCL-injured knees. While the mean force placed through the knee was not significantly different between the healthy and PCL- injured knee, there was a tendency to place less force through the PCL-injured knee. This is an important consideration given that previous studies have suggested standardized values of SSD in PTD associated with varying degrees of PCL insufficiency.^1,7,18,19 Given that standardized values for displacement are being utilized to determine the extent of injury, it is important that the displacing forces be equally standardized such as is seen with the weighted gravity method. Several studies, however, have been published that utilize varying loading forces when determining cutoff values. For example, Garavaglia et al⁵ completed a cadaveric study in which they measured posterior laxity at 80° of flexion after applying a force of 180 N. They found SSD measurements of <6 mm, 7 to 12 mm, and >12 mm to be associated with partially sectioned PCL, completely sectioned PCL, and multiligament sectioning, respectively.⁵ An additional study completed by Hewett et al⁷ utilized a force of 89 N and found that SSD values >8 mm were indicative of complete PCL ruptures with high sensitivity and specificity. This variation in reported displacing forces and injury-specific cutoff values has inhibited the development of clinically useful reference standards.

The large difference in the variability of the kneeling force between patients with deficient PCLs and patients with reconstructed PCLs during the kneeling PCL stress views was unexpected. The etiology of this difference is not clear. Postoperative scar sensitivity and anterior knee pain^2,4,16,20,24 could be a contributor to this variability but would be expected to bias the data toward much less force being placed through the PCL-reconstructed knee when in fact the variability was high in both directions. This pain may also be aggravated by the presence of prominent graft and backup fixation on the proximal tibia as part of the PCL reconstruction technique. Alternatively, given the amount of time required to take the radiographs of both knees (average of ~5 minutes) (Table 2), the patients may have weight-shifted back and forth between the 2 knees to take the pressure off the PCL-reconstructed knee. It is therefore possible that some patients had weight-shifted off their knee during the actual radiograph while others were in a loading phase. Every attempt was made to standardize the instructions to all the patients, regardless of surgical status, with respect to placing an equal amount of force through each knee during the radiography. It is therefore unclear as to why the variability was notably higher in the PCL-reconstructed group. Unfortunately, it is not possible to further investigate this finding post hoc but it does raise concerns with respect to the validity and reliability of using the kneeling PCL stress radiography technique in postoperative patients. This finding therefore requires further study.

In contrast to the kneeling stress technique, the results of the current study indicate that the weighted gravity stress technique is significantly more comfortable for patients. These results provide validation for previous studies that reported that the kneeling technique caused significant pain.^1,9,10 Not only does this pain add to patient discomfort, but it is also associated with the contraction of stabilizing muscles, which may limit the amount of PTD produced.^6,7,10-12,23 With the reduction in pain experienced with the weighted gravity stress technique, patients are less likely to contract the stabilizing muscles, which would otherwise limit the PTD produced. Additionally, the relaxed supine positioning utilized in the weighted gravity technique negates the need for patients to balance themselves during kneeling stress radiographs, further reducing the potential effect of muscle stabilization on posterior tibial translation.

Jung et al¹¹ previously evaluated the time required to take various PCL stress radiographs, but no previous studies have reported on the number of radiographs that were required to obtain adequate lateral images for evaluation. It is therefore not clear as to whether surgeons accept whatever lateral radiographs they get or if the radiography technicians are expected to take multiple images until a perfect lateral is obtained. With the widespread use of digital radiography, this has become much easier and time efficient. Jung et al demonstrated that the gravity stress technique took less time than the kneeling views but they did not discuss the reasons why. The current study has shown that the mean time to perform the imaging was longer than that reported by Jung et al; however, neither the number of images taken nor the start and stop time points were reported by Jung et al. In the current study, both the time required to obtain the images and the number of images required to obtain acceptable quality were similar between the 2 techniques.

The heterogeneous distribution of injury patterns observed in the study population may represent a limitation in the current study. While the study was adequately powered to detect differences in SSD across all patients with PCL injuries, the sample size may have limited the ability to detect differences across subgroups such as isolated PCL injury, multiligamentous injury, and patients with PCL reconstruction. Additionally, the range of injury patterns and management options for this condition poses a challenge when attempting to eliminate confounding variables. A standardized weight of 20 lbs (89N) was used for the weighted gravity stress views in all patients. It is not currently known if this amount of weight was sufficient to overcome the soft tissue tension of the knee and produce maximal posterior tibial translation. Given the previous work by Jung et al¹¹ that demonstrated that the standard gravity technique produced less PTD than the kneeling view, this nonweighted technique was not included in this study so as to avoid unnecessary radiation for our patients. A future weight-ranging study, however, could be performed to help determine the optimal weight for the weighted gravity stress views. The reliable measurement of PTD also relies heavily on the ability to obtain perfect lateral radiographs and unobscured bony landmarks. Patient discomfort during imaging, bony rotation, periarticular fracture/healing, and implants/bony changes due to ligament reconstruction may all affect measurement reliability. While the intra- and interrater reliability values in this study were found to be acceptable, they may have been affected by the previously listed factors as they are lower than those published by Schulz et al¹⁷ for intratester reliability (experienced rater only, r = 0.95) and intertester reliability (r = 0.91). The study by Schultz et al, however, used the more constrained and expensive TELOS system in a population of chronically injured and nonoperatively managed patients.¹⁷ While this study demonstrates a technique that is comfortable for the patient and provides diagnostic abilities comparable with the kneeling stress views, the results have not been directly correlated with the severity of PCL injury.⁷

Conclusion

Weighted gravity PCL stress radiographs produce similar SSD in posterior tibial translation compared with the kneeling stress technique, but do not rely on patient weightbearing and provide significantly better patient comfort. Relative to other instrumented techniques (eg, Telos), this technique is inexpensive, efficient, and requires minimal additional equipment. Clinicians should therefore consider the use of weighted gravity PCL stress radiographs in clinical practice to minimize the pain associated with stress radiography while allowing for accurate decision making.

Footnotes

Acknowledgements

The authors thank Jordyn Sessel, Tanja Harrsion, and Ranae Hoeft for their contributions to data acquisition and technique standardization, as well as their efforts in coordination and implementation of the study.

Submitted June 13, 2019; accepted October 14, 2020.

Presented at the annual meeting of the AOSSM, Boston, Massachusetts, July 2019.

One or more of the authors has declared the following potential conflict of interest or source of funding: R.M. is a paid consultant for Ossur Inc, Smith & Nephew, and Synthes. J.A.G. is or has been a paid consultant for JRF Ortho, Ossur Inc, Conmed Linvatec, and Vericel Inc; has received travel support from Ossur Inc, JRF Ortho, Conmed Linvatec, and Smith & Nephew; has received research funds from Aesculap Biologics, JRF Ortho, and Arthrex; and has received educational support from Pinnacle Orthopaedics. AOSSM checks author disclosures against the Open Payments Database (OPD). AOSSM has not conducted an independent investigation on the OPD and disclaims any liability or responsibility relating thereto.

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