Patella Position Versus Length of Hamstring Muscles in Children,
by M. Jozwlak and S. Pietrzak, Journal of Pediatric Orthopedics, 1998, 18, 268–270.
The authors of this study suggest that a relationship exists between hamstring length and patellar position in children. Citing related literature, they state that shortened hamstring length is often observed in patients experiencing knee pain. They propose that this shortness causes a relative elongation of the patellar tendon, thus resulting in a high-standing patella. Based on this proposal, the authors believe that patellar height is the most sensitive indicator of the existing state of muscle length balance between flexors and extensors of the knee joint. Therefore, the purposes of this study were two-fold: 1) to determine whether a correlation exists between hamstring length and patellar placement using ultrasonographic examination, and 2) to specify the norms of the patellar positioning in the knee joint of children based on ultrasonographic examinations.
The subjects included 57 children, aged four to 16 years, who were being treated at the time for diseases of the upper extremities. The total number of knee joints evaluated was 114. Upon clinical examination, none of the children had extensor contractures of the knee joints, limitations of active or passive knee bending, or any paresis of lower limb muscles. All of the children were able to walk.
Hamstring length was determined by measurement of the popliteal angle (PA) with the child lying in the supine position. The ultrasonographic measurement of patellar height was taken using a linear ultrasound transducer (5 MHz), which was placed in the midline of the knee in the sagittal plane. The length of the patellar tendon and the length of the patella were each measured separately. The degree of knee joint flexion during the measurements depended on the picture of the patellar tendon echo: a straight line on the ultrasonogram was required.
The results of the ultrasonographic measurements were used in the calculation of the tendon-patellar (T-P) coefficient (T-P coefficient = length of the tendon/length of the patella). The calculated values for T-P were then submitted for statistical analysis to determine correlation with age, sex, and the PA. The same calculations were performed for each knee and also for the average values of both knees of the same child.
No significant correlations were found between the T-P coefficient and age or sex. A statistically significant but weak correlation was found, however, between the T-P coefficient and the PA (p < 0.05) when calculated for 114 knees. The Spearman rank correlation coefficient was used to reveal a significant correlation between the T-P coefficient and PA of the 57 pairs of knees of the children (p < 0.02).
The results of this study revealed a relationship between patella position and hamstring length in children. The authors concluded that this correlation “proves biomechanical connections between the elements of the extensor apparatus and knee joint flexors.” Furthermore, they suggest that this correlation indirectly indicates that muscle length balance in the knee joint region is one of the factors affecting patellar height. Upsetting this balance, as can occur in medical conditions such as cerebral palsy, results in incorrect positioning of the patella that may lead to the development of various forms of patellofemoral joint pathologies.
In addition, the authors compared their calculated T-P coefficient norms with those obtained by other researchers who performed ultrasonographic examinations of adults. They concluded that their calculated norms, equal for all age groups, were very similar to the other researchers’.
The authors also listed some suggestions for future research. They proposed research that would focus on children with cerebral palsy to determine whether the high-standing patella is primary or secondary to shortened hamstrings. They also recommended additional assessments to determine whether hamstring length and patellar position are reliable indicators of future development of patellofemoral pathologies.
Limitations and Implications
The authors did not present a thorough review of the literature pertaining to patella position and hamstring muscle length in children. They stated that their thesis was that “the patellar height is the most sensitive indicator of the existing state of muscular balance between flexors and extensors of the knee joint.” However, the authors provide no support for this statement, an omission that detracts from the validity of their study. In addition, the researchers failed to adequately clarify the relationship of this study to others with respect to methodology and results. They wrote the article with the false assumption that anyone reading it would have also read the sources referred to in the article.
The methods were described clearly, thus rendering them easily reproducible. However, no mention was made about the reliability or validity of ultrasonographic measurements, or the PA measurements. Likewise, it was not stated if intertester reliability was controlled during data collection.
The generalizability of this study is limited by the small sample size. Because the authors tested only 57 children between four and 16 years, the norms of the patellar positioning in the knee joint may not be accurate. This does not provide a large enough subject population for the age range specified. By including more subjects in future studies and using a narrower age span, generalizability and specification of norms can be better established.
The correlation value found in this report was very effective at explaining the relationship between the T-P coefficient and PA. However, including a table or figure that shows the data collected for each subject would have been helpful.
Despite these limitations, the authors of this study raised good questions regarding muscle length imbalances at the knee joint region and patellar position as indicators of future patellofemoral joint pathologies. They provided a method of measurement that is easy to use. They also attempted to specify norms of patellar positioning in children. Additional studies are indicated to 1) determine the reliability and validity of ultrasonographic measurements, 2) establish norms of patellar positioning using a larger sample size, and 3) determine the clinical implications of early intervention in children who exhibit muscle length imbalances around the knee joint or improper patellar positioning.