Radiographic Measurements of Normal Knee Joint Space in Adults

Introduction: The knee joint is the space between the femoral and tibial condyles. The tibiofemoral articulations are separated by articular cartilages and menisci; with little contributions from cruciate ligaments. These are collectively seen on a radiograph as a radiolucent area between the bony tibial and femoral condyles. This study was conducted to measure the normal knee joint space in patients referred for X-Ray to the Department of Radiology of a tertiary care hospital. Methods: This descriptive cross-sectional study was performed from 1st January to 1st April 2021 in the Department of Radiology and Imaging, Tribhuvan University Teaching Hospital (TUTH). A total of 157 patients were involved. The radiographic joint space width in the knee joint was measured in both medial and lateral compartments of bilateral knees in correlation with age and gender. Results: The mean right medial, left medial, right lateral, and left lateral spaces for females were 0.25±0.08 cm, 0.23±0.09 cm, 0.49±0.10 cm, and 0.49±0.29 cm respectively. Similarly, the mean right medial left medial, right lateral, and left lateral spaces for males were 0.26±0.07 cm, 0.10±0.09 cm, 0.18±0.09 cm, and 0.17±0.08 cm respectively. There was a strong positive correlation between both right and left joint spaces about age. Conclusion: The radiographic reference values for the joint space width were obtained, showing a positive correlation with age and no significant gender variations in bilateral knees.


INTRODUCTION
The knee is a weight-bearing synovial hinge joint comprising two condyles (between the femoral and corresponding tibial condyles); and a saddle joint (between the patella and the patellar surface of the femur). The tibiofemoral articulations are separated by articular cartilages and menisci; with little contributions from cruciate ligaments. The measurement of the distance between the distal femur and the proximal tibia known as joint space width (JSW), is an indirect way of measuring cartilage thickness. This has become the standard and reproducible tool for the assessment of progressive knee cartilage degenerative conditions with fairly good precision. These are seen on a frontal radiograph as a radiolucent area between the bony tibial and femoral condyles, measuring between 3 to 8 mm. 1,2 Usually, the lateral knee JSW is wider than the medial and males tend to have wider JSW than females. 3 Osteoarthritis (OA), a degenerative disease, is a leading cause of knee joint narrowing. It is the most common joint disorder and is responsible for substantial economic, social, and psychological costs. The majority of individuals over the age of 65 years in developed nations demonstrate radiographic evidence of OA. Osteoarthritis may be idiopathic or secondary to previous inflammation or trauma. One of its earliest signs may be the narrowing of the medial compartments of the joint. This is followed by subchondral sclerosis and cystic changes of the articular surface. As a result of wear and tear over time, there is a gradual reduction in JSW with increasing age. 3,4 The techniques of quantitatively assessing knee joint spaces using, digital radiographs have a low inter-observer coefficient of variation ranging from 1% to 3.3%; and are better than that obtained by the use of dividers and rulers. The computerized measurement of joint space width from standard radiographs is applicable to monitor the progression of knee degenerative conditions and is of value in the evaluation of disease-modifying therapies for osteoarthritis. 4-8 Therefore, we are measuring the JSW in bilateral knees in the adult population in this study thereby correlating the JSW with age and differences between the right and left knees.

METHODS
This was a quantitative, radiographic study conducted in the Department of Radiology and Imaging, Tribhuvan University Teaching Hospital (TUTH) for three months. A purposive nonprobability sampling technique was used in the sampling of the subjects. The adult population above 20 years referred for X-Ray Knee was included in the study. Knee with fractures was excluded from the study. An anteroposterior weight-bearing x-ray was performed. The patient stood in front of the cassette with the posterior thigh in contact with the cassette. Irradiation was applied in the horizontal direction. Measurements were taken of erect bilateral knees in anteroposterior (AP) view projection. 3,9,10 Statistical analysis was carried out with the help of SPSS version 20 and Microsoft Excel 2013. The mean, SD, and comparison of different knee joint space measurements were expressed in tables and bar diagrams. Data were presented as mean and SD for all the variables. The patient's age, gender, right medial space, right lateral space, left medial space and left lateral space were recorded. Data were tested for normal distribution using the Shapiro-Wilk test. Mann-Whitney test, Spearman's correlation, paired t-test, and the pear correlation coefficient was used for statistical analysis. A 95% confidence interval was taken and p<0.05 was taken as statistically significant.

RESULTS
The data was collected from 157 subjects with the age above 20 years. Among them 95(60.51%) were female and 62(39.49%) were male. The mean age group of the sample population was 47.27±13.499 years (Fig. 1).

Figure 1: Bar diagram of the distribution of sample size according to age group
The mean right medial left medial, right lateral, and left lateral for females were 0.25±0.08, 0.23±0.09, 0.49±0.10, and 0.49±0.29 respectively. Similarly, the mean right medial left medial, right lateral, and left lateral for males were 0.26±0.07, 0.10±0.09, 0.18±0.09, and 0.17±0.08 respectively. There was a strong positive correlation between RMS and LMS (r=0.677, p<0.001), which is statically significant at p<0.05. There was also a strong positive correlation between RLS and LLS (r=0.632, p<0.001) which is statically significant at p<0.05.    It represents the articular cartilage and the menisci while the inter-condylar notch of the femur and the inter-condylar eminence of the tibia is seen opposite to one another. The medial and lateral condyles of the tibia, as well as the head and neck of the fibula, appear well corticated. 1,2,7 On the lateral view of the knee joint, the fabella is seen as sesamoid bone in the lateral head of the gastrocnemius. It can be shown behind the lateral condyle of the femur if present. The patella is seen in front of the femoral condyles while the tibial tuberosity is seen on the front of the upper end of the tibia. 1,7,11 3 4 Figure 3: A strong positive correlation was observed between age and RMS (right medial space) with r=0.677 (p<0.001) which is statistically significant at p<0.05.

Figure 4: A strong positive correlation was observed between age and LMS (left medial space) with
r=0.677 (p<0.001) which is statistically significant at p<0.05

DISCUSSION
The knee is a synovial hinge joint between thigh and leg. The articular surfaces include the condyles and the patellar surfaces of the femur, the tibial articular surfaces on the tibial plateau, and the deep surface of the patella. 1,2

Normal Radiographic Anatomy of the knee
The anteroposterior knee radiograph shows the well corticated two femoral condyles and the adductor tubercle. The joint space is the radiolucent area between the articular surfaces of the femur and tibia. Figure 5: A strong positive correlation was observed between age and RLS (Right lateral space) with r=0.632 (p<0.001) which is statistically significant at p<0.05 Figure 6: A strong positive correlation was observed between age and LLS (left lateral space) with r=0.632 (p<0.001) which is statistically significant at p<0.05

6
The head of the fibula is seen overlapping the tibial meta-diaphysis. The intercondylar eminence of the tibia can be identified as the bony elevation at the midpoint of the tibial condyles. The 16 tibial tuberosity is very variable in appearance, particularly during the growth period. Asymmetry and irregularity on radiographs may be quite normal. 7 Subcutaneous and intra-articular fat (the infrapatellar fat pad) outlines some of the soft tissues around the joint. The suprapatellar bursa is best seen on radiographs when distended with fluid. It is then seen as a soft-tissue density above the patella. 1 We measured the knee joint space on two sides (medial and lateral width) on bilateral erect knee AP radiographs of normal adults. The medial and lateral width of knee joint space differs on each side. In our study, the mean right medial left medial, right lateral, and left lateral for females were 0.25±0.08, 0.23±0.09, 0.49±0.10, and 0.49±0.29 respectively.
Similarly, the mean right medial left medial, right lateral, and left lateral for males were 0.26±0.07, 0.10±0.09, 0.18±0.09, and 0.17±0.08 respectively. There was a strong positive correlation between RMS and LMS (r=0.677, p<0.001), which is statically significant at p<0.05. There was also a strong positive correlation between RLS and LLS (r=0.632, p<0.001) which is statically significant at p<0.05. No statistically significant difference between right and left joint space width as well as no statistically significant difference between male and female knee joint space was seen. There are several publications similar to our study. [12][13][14][15][16][17][18][19][20][21] The study by Anas et al. in  They measured the mean left medial and lateral joint space width to be 5.34mm±1.26 mm and 5.21mm±1.04 mm and on the right side 5.26±1.11 mm and 5.57±1.57 mm respectively. No statistically significant differences were found between right and left knee joint space width compartments. From their study, they concluded that there were no statistical differences found between right and left knee joint space width compartments similar to our study.

CONCLUSION
The medial and lateral space of bilateral knee joints were measured in a radiograph of erect knee AP view. Variation exists in radiographic images in the medial and lateral width of knee joint space. No statistically significant difference between right and left joint space width compartments is seen. There is no statistically significant difference between the genders.