The physis (growth plate) of the distal tibia and femoral physis maintain its planar structure throughout gestation [1]. The resting zone, the proliferative zone, and the hypertrophic zone are considered the three zones of the growth plate and are responsible for endochondral ossification at the growth plate [9]. The resting zone contains a high level of extracellular matrix with chondrocytes that are in a quiescent state neighboring the epiphyseal bone. Within the proliferative zone, chondrocytes are organized in cell columns, which are flat in appearance and undergo mitosis. In the hypertrophic zone, terminal cell differentiation occurs and chondrocyte division ends [10]. The femoral and tibial physes are discoid in shape, but as development progresses, the physes gradually decrease in width.

Anderson et al. [11] published work about growth and characteristics of growth plate development. There were several limitations to this work, including the following: (1) they studied a small group of patients with half of the patients having polio in their contralateral knee, and (2) they assumed that there was a constant contribution of each physis during development [12]. Nonetheless, they suggested that the distal femoral physis contributed 70% to the overall femoral length and 40% to overall lower extremity length with the tibia physis contributing 55% and 27%, respectively. The maturation height was 175 cm in males and 162 cm for females. Modern American children maturation height increased slightly to 179 cm (males) and 167 cm (females) [12].

Pritchett [12] reported that the distal femur contributed to an average of 1.3 cm of femoral growth per year and slowed to 0.65 cm during the last 2 years of maturation. His work also showed that the distal femoral and proximal tibia make increasing contributions to growth as age increases. The proportion of femoral growth for girls at the distal femoral physis was 60% at age 7 and 90% at age 14. Boys showed a similar trend with the distal femur contributing 55% at age 7 and 90% at age 16, and activity slowing as they reach skeletal maturity.

The tibial tuberosity does not become discrete until 12–15 fetal weeks [13]. The vasculature of the tuberosity differs from that of the metaphyseal and epiphyseal. Ogden and Southwick further characterized the several stages of development through a continuum model of the tibial tuberosity [13].

The tibial tuberosity physis is divided into three regions that are not easily recognizable. Proximally, the cytoarchitecture is analogous to the remainder of the proximal tibial growth plate and transitions into [14] the fibrocartilaginous zone, which is composed of hyaline cartilage [15], bone forming from membranous ossification, and [11] fibrocartilage. In the distal region, there is a transformation from hyaline cartilage to fibrous tissue, which further transforms into bone through membranous ossification. After the distal extension and growth of the secondary ossification center, the columnar region becomes more distally extended [13].

Stages I, II, and III comprise the prenatal phase. The physis is transversely oriented during stage I with no discrete tibial tuberosity. Stage II is defined by the anterior outgrowth of the tibial chondroepiphysis, and this development occurs simultaneously with the vascularization and fibrovascular ingrowth of the chondroepiphysis. During stage III there is continued fibro-mesenchymal-vascular ingrowth, which causes anatomical separation from the proximal tibial physis. Distal displacement of the tuberosity by longitudinal growth at the proximal tibial physis also occurs at stage III [13].

The postnatal phase includes four subsequent, distinct, developmental stages. Stage IV is characterized by development of an additional growth plate at the tibial tuberosity, which coalesces with the proximal tibial physis. During stage V, a secondary ossification center in the distal portion of the tuberosity develops. At stage VI, the proximal tibial epiphysis and tuberosity ossification centers coalesce (Fig. 3.2). The end of the postnatal phase is marked by the closure of the contiguous growth plates of the proximal tibia and tuberosity (stage VII) [13].

The ACL originates on the lateral femoral condyle (LFC) and inserts on the medial intercondylar spine of the tibia (Figs. 3.3, 3.4, and 3.5). At the lateral and medial insertion sites of the ACL, the area is three times larger than the ligament itself [6]. The femoral origin site has an oval-shaped appearance and is smaller than the tibial insertion site (Fig. 3.3) [16, 17]. As the knee is extended or flexed, the femoral origins change in orientation becoming vertical in extension and horizontal with flexion [7].

Ferretti and colleagues studied the femoral attachment landmarks for both fetal and adult patients [18]. They identified two ridges, the “lateral intercondylar ridge” and the “lateral bifurcate ridge,” which together make up the ACL femoral origin. The lateral bifurcate ridge was present in 6 out of 7 fetuses. Of those six specimens, four had a distinct ridge that separated the two bundles. The lateral bifurcate ridge was better identified in all patients in the anterior aspect of the ACL origin and separated the AM bundle from the PL bundle. ACL insertion did not occur anterior to the “resident’s ridge” and was a mean 17.1 ± 1.2 mm in length and 9.9 ± 0.8 mm wide with a total area of 196.8 mm².

The origin/attachments of the anterior cruciate ligament are in close proximity to the physis, so ACL reconstruction risks iatrogenic physeal injury in the skeletally immature knee (Fig. 3.3). Studies by Shea and Behr have investigated the tibial and femoral origins of the anterior cruciate ligament in relation to the physis in the skeletally immature [4, 19].

Behr et al. [4] examined 12 fetal specimens ranging from 20 to 36 weeks gestation and 13 skeletally immature knees between the ages of 5 and 15. The ACL origin was located distal to the femoral physis in all specimens. Specimens younger than 24 weeks gestation showed a fetal ACL origin, which developed as a confluence of ligament fibers with periosteum. Vascular infiltration into the epiphysis is present at 24 weeks, and the ACL is completely epiphyseal by the age of 36 weeks gestation. The distance from the most superior aspect of the ACL origin to the distal femoral physis in fetal specimens was 2.66 mm and showed no significant change in preadolescent and adolescent specimens at 2.92 mm. In relation to the growth of the femur, there was no significant increase in distance from the ACL origin and the femoral physis.

The coauthors of this chapter conducted an initial study of the anatomical relationship between the ACL femoral origin and the distal femoral physis in eight skeletally immature cadaveric knees [20]. There were two groups in the study: infants (1 and 11 months of age) and children (ages ranging from 8 to 11 years). Metallic markers were placed at the midpoint of the femoral ACL origin, and the distance from the ACL femoral origin to the distal femoral physis was measured using computerized tomography (CT) scans. The infant group had a mean distance of 6.3 mm (range 5.8–6.8 mm) from the ACL origin midpoint to the physis. Group 2 (ages 8–11 years) showed a mean distance of 8.3 mm (range 6.7–9.7 mm) (Fig. 3.6). In contrast to the findings of Behr et al. [4], this study suggested the distance between the ACL origin and the distal femoral physis may increase with growth and maturation.

Shea et al. [21] recently described the relationship of the ACL femoral origin to the most posterior aspect of the lateral femoral condyle and proximal aspect of the posterior physis in a group of 12 pediatric specimens (ages 7–11 years) using superimposed sagittal plane CT images. The median distance from the most posterior aspect of the ossified lateral femoral condyle to the midpoint of the ACL origin was 7 mm (interquartile range, 5–8 mm). This distance represented 14% of the total distance from the posterior aspect of the condyle to most anterior (14% anterior-posterior). The median distance from ACL origin midpoint to the proximal aspect of the posterior physis was 10 mm (interquartile range, 9–10 mm), which corresponded to 38% of the total distance from the proximal aspect of the LFC to the most distal aspect of the LFC (38% posterior-distal) to the total lateral femoral condyle height. The ACL origin was found to lie distal to the distal femoral physis in all cases. Furthermore, the same study provided the 95% confidence intervals of the true mean ACL origins (Fig. 3.7).

The ACL can be separated into multiple bundles, but several groups have grossly separated the ACL into two distinct bundles. The work of Ferretti et al. and others has suggested that separating the ACL into anteromedial and posterolateral bundles is a reasonable distinction for better understanding the anatomic and functional aspects of the ACL [7, 17, 18]. The tibial spine is associated with the two bundles of the ACL, the anteromedial (AM) and posterolateral (PL). The posterolateral bundle inserts into the posterolateral ACL footprint in close proximity to the lateral aspect of the tibial spine, while the AM bundle is located anterior and medial to the PL bundle (Fig. 3.8) [7, 17]. A study conducted by Siebold et al. [22] demonstrated that in relation to the mid-substance of the ACL, the tibial attachment site is larger in diameter and also shows a characteristic “c shape.” Siebold et al. [22] looked at the mid-substance in 20 cadaveric specimens. The mid-substance of the ACL had a “ribbon-like” or “flat” appearance with a width of 9.9 mm and thickness of 3.9 mm. Along the medial aspect of the tibial spine, the ACL insertion showed a close anatomic relationship to the anterior aspect of the anterior root of the lateral meniscus. The width and the thickness of the “c-shaped” insertion were 12.6 mm and 3.3 mm, respectively. In contrast to previous studies [23–27], it was found that the ACL fibers inserted in the anteromedial and posteromedial parts of the tibia, but not the posterolateral part. The authors hypothesized that anatomic ACL reconstruction techniques may require a tibial tunnel resembling the native “c shape” of the ACL footprint.

Previous MRI-based research has measured the anterior and posterior limits, center point, and angle of roof of the ACL in skeletally immature children [19]. For males, in relation to the anterior-posterior tibial width, the values for anterior limit, center point, posterior limit, and roof angle of the ACL were 28%, 43%, 59%, and 36.8°. In relation to adult males, the corresponding values were 27%, 43%, 59%, and a 40° roof angle. The study saw minimal age and gender variability. For skeletally immature females, similar values included 28%, 46%, and 63% and 18° roof angle. Adult female had corresponding values of 28%, 44%, and 60%, with a roof angle of 35° (Fig. 3.9). In the skeletally immature and adult knee, the anatomical landmarks are proportionate regardless of size differences.

As the primary stabilizer of the knee, the ACL is responsible for resisting anterior translation and rotation of the tibia on the femur. During activities of daily living, the ACL experiences minimal stress. The anteromedial and posterolateral bundles that attach to the tibia make up the ACL (Fig. 3.8). The posterolateral bundle originates in close proximity to the anterior articular cartilage of the lateral femoral condyle, while the anteromedial bundle originates proximally in the intercondylar ridge. The anteromedial bundle inserts onto the femur superior to the posterolateral insertion (Fig. 3.3) [7, 17, 18]. Under maximal anterior tibial loads and simulated muscle loads, the ACL has been shown to be maximally loaded within 30° flexion [28]. These bundles function as reciprocals, as the anteromedial bundle is tight at high-flexion angles and the posterolateral bundle is tight at low-flexion angles [29].

Li et al. [28] investigated elongation of the ACL during flexion in an MRI-based study. The length of the anteromedial bundle at 30°, 60°, and 90° of flexion was 32.5 ± 2.8 mm, 30.7 ± 2.0 mm, and 30.2 ± 1.8 mm, respectively, and 32.5 ± 3.7 mm in full extension. The corresponding posterolateral bundle values were 26.3 ± 4.1 mm (30°), 23.5 ± 2.3 mm (60°), 24.1 ± 2.9 mm (90°), and 27.6 ± 5.2 mm (full extension). They found little change in anteromedial bundle length between full extension and 90° of flexion. Consistent lengths of the anteromedial bundle within full extension and 90° flexion may indicate an approximate isometry of the ACL anteromedial bundle.