A method to establish the relationship between chronological age and stage of union from radiographic assessment of epiphyseal fusion at the knee: an Irish population study

Abstract

Characteristic changes during epiphyseal union provide a skeletal age, which when compared with age-based standards provides an estimation of chronological age. Currently there are no data on epiphyseal union for the purposes of age estimation specific to an Irish population. This cross-sectional study aims to investigate the relationship between stage of epiphyseal union at the knee joint and chronological age in a modern Irish population. A novel radiographic method that sub-divides the continuum of development into five specific stages of union is presented. Anteroposterior and lateral knee radiographs of 148 males and 86 females, aged 9–19 years, were examined. Fusion was scored as Stage 0, non-union; Stage 1, beginning union; Stage 2, active union; Stage 3, recent union; or Stage 4, complete union. Stage of epiphyseal union is correlated with chronological age in both males and females. Mean age gradually increases with each stage of union and also varies between male and female subjects. A statistically significant difference in mean age was recorded between stages when compared to the previous stage, for the three epiphyses. Irish children are comparable to those from previously published studies with epiphyseal union in females occurring earlier than males. A significant difference was noted between the mean age of union for males and females for each of Stages 1 and 2 for the femur and Stages 0, 1, 2 and 3 for the tibia and the fibula. The results also suggest that the stages of union occur at earlier ages in this Irish population. Implementation of standardized methodology is necessary to investigate if this is due to a secular or population variation in maturation or to a methodology which clearly identifies five stages of union.

Introduction

Biological anthropologists are concerned with gaining an insight into the characteristics and maturational processes of the human skeleton to develop methods for the construction of biological profiles. Skeletal maturation is the metamorphosis of the cartilaginous and membranous bones of the fetus to the fully ossified bones of the adult (Acheson, 1954: p. 498). By its very nature it is restricted to early life and thus provides an ideal mechanism for the development of methods that estimate age. Information about the time at which epiphyseal union occurs can be used to estimate age between 10 and 20 years, where data on long bone length becomes of limited value (Stevenson, 1924; Ubelaker, 1989).

Available data on completion of epiphyseal union is primarily from archaeological skeletal collections and from radiographic studies of the early to mid-20th century (Table 1). It is clear that differences exist in the timing of epiphyseal union between individuals from different populations (Banerjee & Agarwal, 1998; Schaefer & Black, 2005). This difference may be a consequence of population variability, secular changes (Eveleth & Tanner, 1990; Lampl & Johnston, 1996) or simply lack of standardized methodology. Given that practical situations requiring the establishment of age including clinical and forensic investigations, call for high levels of accuracy, it is critical to revise these data upon which reliance has been placed (Ritz-Timme et al. 2000). In the absence of large collections of contemporary skeletal collections of immature human remains, radiographs provide an alternative avenue of analysis.

Table 1.

Summary of previous studies providing age ranges for completion of epiphyseal union at the knee

					Males			Females
Author	Year	Sample size	Population	No. of stages	Femur	Tibia	Fibula	Femur	Tibia	Fibula
Stevenson	1924	90(M) 20(F)	US	4	19	19	19	19	19	19
Davies & Parsons	1927	N/S	UK	2	19	19–20	20–22	19	19–20	20–22
Hepworth	1929	83	Indian	2	16.5–17.5	16.5–17.5	16.5–17.5	16.5–17.5	16.5–17.5	16.5–17.5
Paterson	1929	< 100	UK	2	18	18–19	18	16–17	16	16–17
Todd	1930	N/S	US	9	17.5–18.5	17.5–18.5	17.5–18.5	17.5–18.5	17.5–18.5	17.5–18.5
Flecker	1932	70(M); 38(F)	Australian	2	16–19	16–19	16–19	14–19	14–19	14–18
Pillai	1936	100	Indian	N/S	14–17	14–17	14–17	14–17	14–17	14–17
Galstaun	1937	N/S	Indian	2	> 18	16–17	16	> 17	14–15	16
Flecker	1942	76(M); 41(F)	Australian	2	16–19	16–19	16–19.66	14–19	14–19	14–18
Agarwal & Pathak	1957	95(F)	Indian	N/S	–	–	–	14.5–16.5	14.5–16.5	15–16.5
McKern & Stewart*	1957	450(M)	US	5	22	23	22	–	–	–
Narayan & Bajaj	1957	300(M); 25(F)	Indian	2	18–19	18–19	18–19	16	16	16
Johnston*	1961	35(M); 27(F)	American Indian	3	18.5†	18†	18†	17†–18	16†–18	16†–19
Hansman	1962	102(M); 105(F)	US	2	14–19	14.5–19.5	15–20	12–17	12–17	12–17
Saksena & Vyas	1969	50(M); 25(F)	Indian	2	18–19	18–19	18–19	16–17	16–17	16–17
Das Gupta et al.	1974	44(M); 31(F)	Indian	2	18–19	18–19	20–21	17–18	17–18	20–21
Pfau & Sciulli	1994		US	3	–	–	–	–	–	–
Schaefer & Black*	2005	114(M)	Bosnian	5	17–20	17–20	17–20	–	–	–

N/S – not specified.

^*Data based on the assessment of skeletal remains.

^†This figure represents the beginning of union and not the lower range for completion of union.

The hand-wrist region has received the greatest attention in assessment of skeletal maturation (Greulich & Pyle, 1959; Lee, 1971). It is the maxim of age estimation that we should combine information from as many epiphyses as possible to provide the most accurate estimate of biological age and therefore chronological age. By diverting research on maturation to other areas we can provide new information that has the possibility of being used in combination with these well-established techniques to maximize the accuracy of age estimation. The knee is an ideal anatomical locus for assessment of epiphyseal union. Clinically the patient's knee joint often presents for investigation of trauma. As a result there is a large databank of knee radiographs available. The knee is easily positioned so that anteroposterior radiographs can be taken and yields information for three epiphyses – the distal femur, proximal tibia and proximal fibula. This research presents a revised method for radiographic assessment of epiphyseal union in a cross-sectional investigation of the relationship between stage of epiphyseal union at the knee joint and chronological age in a modern Irish cohort. This study aims to provide a method to enable future researchers to arrive at more uniform results and to provide a detailed picture of the range of variation in epiphyseal union in an Irish population.

Materials and methods

Sample

This retrospective cross-sectional analysis of the relationship between epiphyseal union at the knee and chronological age was undertaken using a sample of radiographs from Cork University Hospital (CUH), Ireland. Based on the results of a preliminary investigation, all subjects aged 9–19 years who presented for an x-ray of the knee through the Accident and Emergency or Out-patient Departments at CUH during 2002 were retrospectively selected for inclusion. Consequently, sample sizes were not equal across all age ranges from 9–19 years. All subjects were from families who have a history of residence in Ireland. The chronological age of each subject was calculated using information provided on the date of birth and the date of registration for x-ray, therefore allowing calculation of exact age (years) at the time of x-ray. The initial number of subjects recruited to the study was 341. Exclusion criteria were as follows: individuals who were not resident in Ireland, subjects who had a previous knee x-ray as trauma may have influenced the rate of epiphyseal union, subjects who had only one radiographic view of the knee, individuals exhibiting trauma or pathology of the knee, and subjects with known endocrine, metabolic or nutritional disorders. In all, 107 subjects were not included in this study based on these criteria. The final number of subjects included in the study was 234 (148 males; 86 females). All individuals had an anteroposterior and a lateral radiograph of the knee available for assessment.

Revised method for assessment of epiphyseal union at the knee

The literature does not support a consensus view on the definable stages of change between no mineralized bridge formation, non-union, and complete ossification. McKern & Stewart (1957) sub-divide the continuum of epiphyseal union at the knee into five stages; non-union, beginning union, active union, recent union and complete union. Even though these stages were derived from morphological specimens they represent stages in the process of union that may be adapted for use with radiographic assessment. During a preliminary study the designations of stages used by McKern & Stewart (1957) were adapted to produce the revised method outlined. This involved the examination of a sub-sample of knee radiographs and a review of current literature on epiphyseal union. This provided written descriptions and images specific to the knee, of each of the five stages of union identified.

Criteria for assignment of stage of union

Stage 0 – non-union (Fig. 1a,b)

Fig. 1.

Plain film radiograph of a subject's knee showing (a) anteroposterior view demonstrating non-union (Stage 0) of the epiphyses and (b) lateral view confirming non-union.

The diaphyseal and epiphyseal bones are adjacent to each other but as yet show no intimate relation. The epiphysis remains separate from the diaphysis due to the presence of the cartilaginous growth plate. This is apparent on the radiograph, in at least one view, as a continuous radiolucent gap between the epiphysis and diaphysis. The terminal plate is a layer of cortical bone, on the metaphyseal aspect of the epiphyseal centre of ossification that is visible radiographically (Roche et al. 1975). The terminal plates are present and complete. This represents a boundary of the radiolucent growth plate clearly demarcating it from the metaphysis.

Stage 1 – beginning union (Fig. 2a,b)

Fig. 2.

Plain film radiograph of a subject's knee showing (a) anteroposterior view demonstrating beginning union (Stage 1) of the epiphyses and (b) lateral view confirming beginning union.

The epiphyseal and diaphyseal surfaces closely approximate each other. The radiolucent strip between adjacent surfaces of the epiphysis and diaphysis has become narrowed by comparison with the stage of non-union. The radiolucent gap is not continuous from anterior to posterior or medial to lateral. Towards the central region of the growth plate there is a definite breaking up of the adjacent outlines of the epiphyseal and metaphyseal margins. This appears as a hazy area centrally which is of greater density than the adjacent bone. The radiodense appearance on the radiograph is a consequence of the extension of bone across the intervening gap between shaft and epiphysis. Greater than half the growth plate is judged to be radiolucent. This indicates that union has begun centrally but has not as yet commenced on the remainder of the growth plate. In the event that an investigator is uncertain about what stage to assign after assessing both radiographic views, the later stage should be selected. For example, if an investigator cannot decide between Stage 1 and 2, then Stage 2 should be selected.

Stage 2 – active union (Fig. 3a,b)

Fig. 3.

Plain film radiograph of a subject's knee showing (a) anteroposterior view demonstrating active union (Stage 2) of the epiphyses and (b) lateral view confirming active union.

The epiphysis and diaphysis now cap each other. ‘Capping’ refers to the way in which the epiphysis overlaps the metaphysis as maturation proceeds (Roche et al. 1975: p. 114). The terminal plate of the epiphysis is no longer distinguishable. A fusion line or zone of greater density than the adjacent bone replaces the epiphyseal cartilage. The presence of the radiodense region indicates that fusion is actively occurring. Small areas of radiolucency separating the epiphysis and diaphysis may be evident towards the margins of the bone. Less than half the growth plate is judged to be radiolucent. This indicates that union is actively occurring but has not yet reached the margins of the bone.

Stage 3 – recent union (Fig. 4a,b)

Fig. 4.

Plain film radiograph of a subject's knee showing (a) anteroposterior view demonstrating recent union (Stage 3) of the epiphyses and (b) lateral view confirming recent union.

The epiphysis and diaphysis have united to form a single unit of bone, i.e. there is complete capping. The position of the former epiphysis and diaphysis can still be distinguished. A fine line of fusion of greater density may remain between the epiphysis and diaphysis. There is discontinuity of trabeculae between the former epiphysis and diaphysis. Although the epiphysis caps the diaphysis there may still be a slight notch at the margin of the growth plate (less than 2 mm) that has not yet completely calcified. The combination of discontinuity of trabeculae between the epiphysis and diaphysis and the notches at the peripheral margins of the bone indicate that the bone is recently united.

Stage 4 – complete union (Fig. 5a,b)

Fig. 5.

Plain film radiograph of a subject's knee showing (a) anteroposterior view demonstrating complete union (Stage 4) of the epiphyses and (b) lateral view confirming complete union.

The epiphysis and diaphysis are united as a single unit of bone. Remodelling has taken place and there is continuity of trabeculae from shaft to former epiphysis. This presents as a uniformity of internal bone pattern throughout the end of the long bone up to the articular surface. All trace of epiphyseal differentiation has been lost. There are no radiolucent notches evident at the peripheral margin of the bone, indicating that the growth plate has now completely ossified and the bone is fused in its entirety. A thin terminal line, the epiphyseal scar, which marks part of the location of the former epiphyseo-metaphyseal junction may remain in some cases.

Methodology

Training for this method involved instruction using the written criteria and representative images for each of the stages of epiphyseal union. The authors emphasize that both anteroposterior and mediolateral radiographs should be used together and not separately scored when assessing the stage of epiphyseal union. Where there is a difference in the stage of union between radiographic views, the radiograph of the growth plate which demonstrates the least mature view should be selected. For example, in the case of the distal femur, due to the morphology of the epiphysis, fusion may appear to have commenced in the anteroposterior view. However, examination of the lateral view may clearly demonstrate a continuous radiolucent gap between epiphysis and diaphysis, indicating that union in fact has not commenced. Researchers found the method simple to learn and apply.

Investigators were blinded to information on ethnicity, sex and age of the subject being assessed. Based on the combined use of these criteria the investigators were asked to assign a stage of union for each of the epiphyses of the distal femur, proximal tibia and proximal fibula. One researcher examined intra-observer error. Stage of union for each of the epiphyses for each subject was recorded on three separate occasions. Interobserver error was assessed based on a random sub-sample of radiographs, which were assessed by three investigators.

Statistical analysis

Results were analyzed using SPSS. Analyses included the use of t-tests and correlation tests.

Results

Of the 148 males and 86 females, 16 subjects (eight male, eight female) had radiographs of both left and right knees taken concomitantly. An examination of these 16 subjects found no statistically significant difference in the stage of fusion between right and left sides for the three epiphyses. In the case of the tibia only, there was a difference in stage of union assigned in two instances, and then only by one stage. As a result, all further statistics were based on the combined samples of right- and left-sided radiographs. Where both sides were available, only left sides were included for the purposes of statistical analysis to avoid duplication of results.

Age distribution of the sample

The distribution of subjects at each stage of union for each age group is presented in Table 2 for males and in Table 3 for females. For each of the three epiphyses for both males and females, the progression of epiphyseal union through a number of definite stages of union with increasing chronological age is evident.

Table 2.

Numbers of individuals at each stage of union for each of the distal femur, proximal tibia and fibula, for each age group (years) for male subjects

		Femur Stage of union					Tibia Stage of union					Fibula Stage of Union
Age (years)	n	0	1	2	3	4	0	1	2	3	4	0	1	2	3	4
9–9.9	3	3	–	–	–	–	3	–	–	–	–	3	–	–	–	–
10–10.9	3	3	–	–	–	–	3	–	–	–	–	3	–	–	–	–
11–11.9	4	4	–	–	–	–	4	–	–	–	–	4	–	–	–	–
12–12.9	14	5	9	–	–	–	7	7	–	–	–	13	1	–	–	–
13–13.9	12	1	9	2	–	–	2	8	2	–	–	8	3	1	–	–
14–14.9	21	–	12	8	1	–	2	10	8	1	–	8	9	4	–	–
15–15.9	21	–	7	14	–	–	–	7	12	2	–	6	7	8	–	–
16–16.9	37	–	1	24	12	–	–	1	22	14	–	1	5	21	8	2
17–17.9	25	–	–	4	21	–	–	–	2	21	2	–	–	5	18	2
18–18.9	8	–	–	1	6	1	–	–	–	8	–	–	–	1	5	2
	148

Table 3.

Numbers of individuals at each stage of union for each of the distal femur, proximal tibia and fibula, for each age group (years) for female subjects

		Femur Stage of union					Tibia Stage of union					Fibula Stage of union
Age (years)	n	0	1	2	3	4	0	1	2	3	4	0	1	2	3	4
9–9.9	1	1	–	–	–	–	1	–	–	–	–	1	–	–	–	–
10–10.9	10	8	2	–	–	–	8	2	–	–	–	10	–	–	–	–
11–11.9	8	2	4	2	–	–	2	4	2	–	–	6	2	–	–	–
12–12.9	4	–	4	–	–	–	–	4	–	–	–	2	2	–	–	–
13–13.9	8	–	3	5	–	–	–	3	5	–	–	–	7	1	–	–
14–14.9	10	–	3	5	2	–	–	3	3	4	–	2	2	5	1	–
15–15.9	11	–	–	2	9	–	–	–	–	11	–	–	–	4	5	2
16–16.9	10	–	–	2	7	1	–	–	–	9	1	–	–	2	8	–
17–17.9	14	–	–	–	14	–	–	–	–	13	1	–	–	–	12	2
18–18.9	10	–	–	–	9	1	–	–	–	9	1	–	–	–	4	6
	86

Table 4 presents the chronological age (years) of the youngest and oldest subjects recorded at each stage of union for each of the three epiphyses at the knee for males and females. Mean age (years) and the standard deviation for each stage of fusion are also presented. For example, in males the youngest subject recorded as having beginning union of the distal femoral epiphysis was aged 12.0 years, and the oldest subject 16.2 years. However, the mean age of male subjects demonstrating beginning union was 14.0 years. The numbers of subjects at stage 4 are low but have been included for completeness. It is intended to collect data for older subjects in future studies, which will supplement the information yielded by the current study. Although there may appear to be large age ranges for each stage of union, the mean age provides an indication of the typical age at which each stage of union occurs, while the inclusion of the age of the youngest and oldest demonstrates the variability inherent to the population. This variability is also demonstrated in Fig. 6. The mean ages show a gradual increase with each stage of union and also vary between male and female subjects. A statistically significant difference in mean age was noted between each stage when compared with the stage previous to it (P < 0.05), with the exception of between Stages 3 and 4 for all three epiphyses in males. In females there was no difference in mean age between Stages 1 and 2 for the tibia and between Stages 3 and 4 for both the femur and the tibia. The proximal fibular epiphysis can also be seen to commence union at a later stage than the epiphyses of the femur and tibia. Six male and 10 female subjects were recorded as having reached the state of completed union of the fibula by comparison with one and two subjects for the femur, and two and three subjects for the tibia, respectively.

Table 4.

Mean (SD) and range in age (years) for each stage of union at each of the epiphyses at the knee for males and females

	Males						Females
			Age (years) of youngest (a) and oldest (b) showing fusion					Age (years) of youngest (a) and oldest (b) showing fusion
	n	Stage of union	(a)	(b)	Mean age (years)	SD	n	(a)	(b)	Mean age (years)	SD
Distal femur	16	0	9.0	13.7	11.2	1.25	11	9.5	11.6	10.6	0.59
	38	1*	12.0	16.2	14.0†	1.12	16	10.2	14.6	12.5†	1.47
	53	2*	13.7	18.1	15.9†	1.00	16	11.6	16.9	14.1†	1.37
	40	3	14.7	18.9	17.2†	0.80	41	14.1	18.8	16.9†	1.24
	1	4§	18.5	18.5	18.5‡	–	2	16.4	18.0	17.2‡	1.14
Proximal tibia	21	0*	9.0	14.5	11.8	1.51	11	9.5	11.6	10.6	0.59
	33	1*	12.0	16.2	14.1†	1.12	16	10.2	14.6	12.5†	1.47
	46	2*	13.7	17.8	15.7†	0.97	10	11.6	14.4	13.4‡	0.96
	46	3*	14.7	18.9	17.2†	0.85	46	14.0	18.8	16.7†	1.32
	2	4§	17.2	17.4	17.3‡	0.11	3	16.4	18.0	17.4‡	0.90
Proximal fibula	46	0*	9.0	16.2	13.0	1.77	21	9.5	14.3	11.2	1.19
	25	1*	12.9	16.4	14.9†	0.95	13	11.6	14.6	13.3†	0.93
	40	2*	13.7	18.1	16.2†	0.95	12	13.7	16.9	15.0†	1.01
	31	3*	16.1	18.9	17.4†	0.72	30	14.7	18.8	16.8†	1.06
	6	4§	16.6	18.5	17.4‡	0.71	10	15.3	18.8	17.7†	1.26

^*A significant difference in mean age between males and females was recorded at this stage of union (P < 0.05).

^†A significant difference was noted in mean age when compared to the previous stage of union (P < 0.05).

^‡No difference in mean age was recorded at this stage when compared to the previous stage of union (P > 0.05).

^§The numbers of subjects at Stage 4 are low but have been included for completeness.

Fig. 6.

Age ranges with the mean age, for each stage of union for males and females for (a) femur; (b) tibia and (c) fibula. The lower line on each figure at each stage represents the data for females. The upper line represents the data for males.

Stage of union vs. chronological age

Figure 6(a–c) graphically represents the relationship between chronological age and stage of union for each of the femur, tibia and fibula, respectively. The age range and mean age for each stage of union is presented. The lower line on each figure at each stage represents data for females, the upper line data for males. The figures may be used as a tool to predict chronological age, where a radiograph is presented from an unknown subject. The graph for the fibula varies from those of the femur and the tibia in that the overall time period for development of the fibula is narrower. Multivariate analysis of this data will be presented in future publications.

A correlation test was used to examine the relationship between chronological age (years) and stage of union. The best-fit line for the correlation between age (years) and stage of union for each of the femur, tibia and fibula, for males is presented in Fig. 7(a) and in Figure 7(b) for females. The fibula would appear to commence union after the femur and the tibia for both males and females, yet reaches completion at approximately the same time. This supports the earlier observation that the fibula has a different rate of epiphyseal union to the femur and tibia. The correlation between age (years) and stage of union of the femur, tibia and fibula in males produced correlation coefficients of 0.868, 0.858 and 0.802 (P < 0.01), respectively. In the female group, the correlation test examining the relationship between age (years) and stage of fusion of the epiphyses of the femur, tibia and fibula gave coefficients of 0.880, 0.877 and 0.910 (P < 0.01), respectively.

Fig. 7.

Best-fit line of correlation between age (years) and stage of union for males (a) and females (b).

Males vs. females

A t-test for independent samples investigated if there was a difference in mean age between males and females for each stage of union. A difference in mean age of 0.6, 1.5, 1.8 and 0.3 years, was noted between males and females for each of Stages 0, 1, 2 and 3 for the femur. In the case of the tibial epiphysis the difference in mean age between males and females was 1.2, 1.6, 2.3, 0.5 and 0.1 years for each of Stages 0–4, respectively. The difference in mean age between males and females for each of Stages 0, 1, 2, 3 and 4 for the fibula was 1.8, 1.6, 1.2, 0.6 and 0.3 years, respectively. A significant difference was noted between the mean age (years) of union for males and females for each of Stages 1 and 2 for the femur (P < 0.01) and Stages 0, 1, 2 and 3 for the tibia and the fibula (P < 0.05).

A paired t-test was used to investigate if there was matching between the stages of fusion of the three epiphyses within the knee. It was noted that the femoral and tibial epiphyses in males exhibited similar rates of fusion. There is no similarity in rate of union between any of the three epiphyses in females (P < 0.01).

Observer error

Intra-observer error was examined by one author and is based on observations of 210 radiographs made on three separate occasions. Of 630 observations in total, variation in the stage of union assigned occurred in 19, 16 and 26 instances for the femur, tibia and fibula, respectively. In those cases where there was a difference in stage of union assigned, the variation was by one stage of union in all except one case. In this instance the fibula was involved, and the stage of union assigned differed by two stages.

Interobserver error was assessed using a sub-sample of 50 radiographs, which were assessed by three investigators on one occasion. Of 150 observations, variation in the stage of union assigned occurred on 14, 11 and 7 occasions for the femur, tibia and fibula, respectively. Where differences occurred, the stage of union varied only by one stage.

Discussion

At present, clinical and forensic investigations call for techniques that can provide estimates of chronological age. Constant revision of the techniques in practice must be undertaken to maintain the accuracy of these estimations of age. Radiological methods of examining the epiphysis provide a means of overcoming the deficiency of non-adult skeletal collections and afford us contemporary information on epiphyseal union. It also provides the facility with which large numbers can be examined. As examinations are made on living subjects the chronological age can be accurately established, as also can the subject's personal history. Ideally, a longitudinal assessment of epiphyseal union would yield the most accurate results. However, due to the health risks posed by repeated radiological examinations this is not ethically possible. This problem may be overcome by undertaking a cross-sectional study that examines larger numbers of individuals in each age group.

The fundamental basis of age estimation techniques in juveniles is that the skeleton is not static and is in fact constantly changing in small increments until the adult state is reached. (Mellits et al. 1971: p. 381). However, it appears that rather than examining the process as a whole, there has instead been a focus on the endpoint of development. The majority of authors describe only two stages of union: non-union or completed union. Criteria for union have either not been presented or have generally been defined as ‘entire disappearance of the epiphyseal plate’. Therefore, interpretation is likely to vary among investigators. Consequently, what is presented in the literature is a range for the endpoint of biological maturation and not the range for when epiphyseal union commences and ends for a particular epiphysis.

Despite an apparent wealth of knowledge, it is extremely difficult to find consensus on ages of epiphyseal union at the knee (Table 1). The practice varies regarding what defines the age when epiphyseal union is complete. Johnston (1961) refers to fusion as the age category at which 50% or more of the group show complete union. Saksena & Vyas (1969) place age of complete union at the youngest age group showing greater than 85% of cases united, whereas Das Gupta et al. (1974) classify the age of complete union when 100% of cases are united. There is visible inconsistency between authors in providing a range in age for time of complete union. As a result it makes comparison between the age ranges provided quite difficult.

McKern & Stewart (1957) utilized the information that ordered changes occur at the growth plate during epiphyseal fusion. This was applied to produce a method that divides the process of epiphyseal union into five different stages. Schaefer & Black (2005) subsequently adapted this method for use on a skeletal sample from a Bosnian population. It was assumed that if the continuum of development could be divided into anatomical specimens, then similarly this process of epiphyseal union could be divided on radiographs. No radiographic study of the knee has previously undertaken this. This study is also the first assessment of epiphyseal union at the knee to be undertaken in a modern Irish population.

As McKern & Stewart (1957) and Schaefer & Black (2005) are the two studies with the greatest similarity in methodology to the current study, comparison will be made with these studies insofar as is possible. It must be noted, however, that both of the studies are based on the analysis of skeletal remains rather than radiographs. In addition, both of these samples comprise male subjects aged greater than 17 years at time of death. As a result, information on epiphyseal union is not available for groups younger than 17 years and is also lacking for female subjects.

McKern & Stewart (1957) report instances of non-union in subjects aged 19 years and Schaefer & Black (2005) record an instance of non-union in an individual aged 18 years, for each of the three epiphyses at the knee. Within the Irish cohort where non-union of an epiphysis was recorded, no subject reached a similar age. A 16.2-year-old male was the oldest subject recorded with non-union (Stage 0) of an epiphysis, the proximal fibula.

Schaefer & Black (2005) report instances of beginning union in subjects aged 18 years for each of the three epiphyses at the knee, and McKern & Stewart (1957) reported beginning union in subjects aged 18 years for each of the femoral and tibial epiphyses. Again, no subject from the Irish population was recorded as having beginning union of an epiphysis in this age range. The oldest males demonstrating beginning union in the Irish population were aged 16 years. This would appear to suggest that union commences at an earlier age in the Irish population. However, there is no record of where the central tendency for beginning-union lies in either the American or Bosnian populations as the samples are restricted to subjects older than 17 years. McKern & Stewart recorded a single subject as having beginning union for each of the femoral and tibial epiphyses. Only three, one and two subjects from Schaefer & Black's cohort for each of the femur, tibia and fibula respectively, demonstrated beginning union. These subjects may in fact represent outliers of natural variation occurring within their respective populations, rather than being representative of the time of commencement of union within these groups. Johnston's (1961) examination of a pre-historic American Indian population is the only other study to provide data on commencement of union. In this cohort union of the three epiphyses at the knee begins at 18–18.5 years in males and at 16–17 years in females. By comparison it appears that beginning union occurs at an earlier age in both males and females from the Irish population sample. We cannot, however, conclude whether this is a result of population-specific differences, secular variation or the disparity in methodology.

As the upper limit of the current study was 19 years, it is not possible to comment on the extent of variability in the occurrence of Stage 3, recent union, in male subjects older than this. The youngest male subjects in the Irish population recorded as having recent union were aged 14.7 years for the femur and tibia and 16.1 years for the fibula. The ages reported by both McKern & Stewart (1957) and Schaefer & Black (2005) were older by comparison. The upper extent of the age ranges for recent union from the Irish group, at 18.9 years, does, however, fall within the limits reported by the other two authors. While it appears that the subjects from this Irish cohort may be demonstrating recent union at an earlier age than the American and Bosnian groups, the lack of subjects younger than 17 years in both of these studies prevents confirmation.

Complete union is the only stage of union that is comparable to all previous radiographic studies (Table 1). However, there is still likely to be some inaccuracies due to the aforementioned variations between authors in the classification of complete union. In the current study the youngest male subjects demonstrating complete union of the femur, tibia and fibula were aged 18.5, 17.2 and 16.6 years, respectively. The youngest female subjects having reached complete fusion of the femoral, tibial and fibular epiphyses were aged 16.4, 16.4 and 15.3 years. The age of subjects who have reached completed union within the current study is within the ranges of previously published studies. However, it would appear that the fibula lies outside this range in some cases. This may be explained, however, on the basis of how the authors have classified their range of union, i.e. whether they are classifying it as 100% of subjects demonstrating completed union, in which case some subjects may fall outside this range. This is one of the issues with classification of union as it misses out on some of the variability that is occurring in the main population. The ability of the current method to clearly identify five stages of epiphyseal union on radiographs is a novel technique that will allow more refined age ranges to be produced. Employment of this method for assessment of epiphyseal union as standard protocol in future research may afford answers to the question as to whether the differences in age for epiphyseal union between various studies can be explained by population or secular variation?

Intra-observer error was higher for the fibula than for the tibia and femur; however, interobserver error for the fibula was comparatively low. Visualization of the fibula may be influenced by position of the knee in some instances. The random sub-sample used in the assessment of interobserver error may have had the fibula positioned such that it was clearly visible and therefore less likely to result in a discrepancy between observers. This is supported by the fact that on re-examination of those cases that differed between observers (n = 7) only one of these showed a within-observer difference. Given the significantly larger overall sample size that was assessed for intra-observer error, it is likely that there were more radiographs within the sample where the fibula may have been partially obscured due to positioning, thus resulting in a higher intra-observer error. It is intended to investigate this further in future studies.

A number of subjects in this cohort had radiographs of both knees available for assessment. It was noted that there was no difference in the stage of union of the various epiphyses between right and left sides. The absence of bilateral asymmetry is consistent with previous studies, which indicate that epiphyseal union appears to occur at the same time on both sides (Albert & Greene, 1999). This information is extremely useful in the event that only the epiphyses of one side are available, such as may occur in some forensic investigations. In addition, it means that samples can be maximized in future studies, as authors do not need to select for a particular side.

The current study found that females are consistently developing at a younger age than their male counterparts. This is in agreement with the results of previously published studies, which have found that females typically develop approximately 2 years in advance of males (Paterson, 1929; Flecker, 1932, 1942; Narayan & Bajaj, 1957; Hansman, 1962; Saksena & Vyas, 1969). This study has noted that there is generally a mean difference of 1.5 years between males and females (Table 4). It is clear that separate age range standards are required for males and females to ensure the greatest accuracy of age estimation. Adjustment would need to be made to these age ranges in the event that a subject's sex is ambiguous, as is often the case in non-adults.

In males the strongest relationship between epiphyseal union and chronological age was for the femur, whereas in females the fibula had the strongest relationship with age. This indicates that of all three epiphyses, these epiphyses alone will yield the greatest accuracy for age prediction. When applying this information to the prediction of chronological age from assessment of epiphyseal union, the authors recommend the use of the charts presented in Fig. 6. These charts present the entire extent of variability, as well as the mean age for each of the epiphyses at each stage of union for males and females and therefore are representative of the actual population.

The authors found while undertaking assessment of the radiographs that the stages of union were similar between the femur and the tibia. What becomes apparent is that the femur and tibia have a similar relationship between stage of union and chronological age. In the case of males, the fibula commences union later than either of the other two epiphyses and reaches maturity at approximately the same time (Fig. 7a). A slight variation occurred in females. Similar to males, the fibula in females begins to unite later than the femur and the tibia. However, in the case of females the best-fit line cuts across the lines representing the relationship between the femur and the tibia (Fig. 7b). This indicates that the fibula commences union after the femur and the tibia and as it matures it proceeds at a greater rate and actually reaches the state of complete union in advance of the other two epiphyses. The reason for this is unknown. It may be related to the fact that the fibula is non-weight bearing or it may be related to the fact that the distal femur and the proximal tibia are the greatest contributors to long-bone length in the lower limb. In addition, it is clear that while there is a difference in the time at which the process of epiphyseal union occurs in males and females, once the process is underway it occurs at a similar rate for both males and females. This is supported by the correlation coefficients of 0.868 and 0.858 for males and 0.880 and 0.877 for females for the femur and tibia, respectively, which are almost equivalent. There is slight variation for the fibula, which warrants further investigation.

Conclusions

Currently there is an obvious lack of standards for assessment of epiphyseal union for the purposes of assignment of chronological age. Techniques that are in use cover only the endpoint of maturation and are inconsistent between authors, making comparison between various studies difficult. It is important to eliminate this inconsistency and to standardize the methodology for assessment of epiphyseal union. The current study is the first to present a radiographic technique, which sub-divides the process of epiphyseal union into five stages of fusion. The presence of three further stages between the incomplete and complete stages provides more defined age range estimations for the process of epiphyseal fusion at the knee.