Siffert (1956) analyzed the effect of a transphyseally placed 2.0 mm K-wire on the medial epiphysis of the proximal tibia in rabbits. In none of the animals growth changes could be noted. The wire grew distally from the physis and the cartilage resulting from the drill injury of the growth plate was replaced by bone. In 1960, Johnson and Southwick (1960) drilled bone tunnels (∅ 2,8 mm, corresponding to approximately 11% of the surface of the physis) through the distal femoral growth plate of 6 weeks old rabbits. In all of their 32 cases, they noticed the formation of a transphyseal bone bridge which induced a complete or a partial closure of the growth plate in 3 animals. In 21 cases the bone bridge was partially resorbed whereas resumption was complete in 8 rabbits.

These localized bone bridge formations must be differentiated from the bony transformation of the growth plate occurring at the end of the growth period. At this stage, the entire growth plate cartilage is progressively replaced by cancellous bone (Synder et al. 1994).

The impact of the transphyseal bone bridge on longitudinal growth is depending on several variables. Small bone bridges may be resorbed without influencing the remaining growth process. The absence of a macroscopic growth abnormality does not mean that there are no microscopic changes of the growth plate after surgically or traumatic induced injuries. Guzzanti et al. (1994) found that microscopic changes occurred within the growth plate after injury. They found a hyperplasia of the growth plate and a loss of the classical cellular architecture of this specific cartilage. This could be confirmed by other studies (Seil 2002). Filling a bony tunnel completely by a tendon graft could not prevent the formation of a bone bridge on a systematic basis. On the contrary, an incomplete filling of the tunnel by a tendon graft of a lesser diameter did not systematically result in the formation of a bone bridge (Seil 2002).

Stadelmaier et al. (1995) analyzed the effects of filling transphyseal bone tunnels with tendon grafts in 10 weeks old dogs. Leaving the tunnel (∅ 4 mm) empty resulted in the formation of a transphyseal bone bridge as early as two weeks postoperatively. When the tunnel was filled with a soft tissue graft bridging could be avoided. In the study the tunnels were located in the center of the proximal tibia and in the distal femur. Growth abnormalities could not be found over a four-month observation period. Janarv et al. (1998) implanted a free tendon autograft (patellar tendon or Extensor digitorum longus tendon) into distal femoral tunnels (∅ 1,7; 2,5 and 3,4 mm) in rabbits. Bone bridge developments could be diminished through the implantation of the tendon grafts and growth abnormalities could be avoided. Österman (1994) created large surgical defects in the central part of distal femoral epiphysis on 37 rabbits. If the injured physis was left without grafting, it lead to significant growth disturbance. After filling the large growth plate defect with a fatty tissue augmentation at least a partial regeneration of longitudinal growth was restored and no significant deformities of the joint contours were seen.

Based on early clinical experiences with permanent or temporary epiphysiodesis (Phemister1933; Blount and Clarke 1949) many experimental articles were published in the mid-20th century with the goal to improve growth regulation of long bones, but also to evaluate the risk of epiphyseal fractures or epiphysiolysis on the remaining longitudinal growth (Haas 1948, Haas1945a,b, 1950; Campbell et al. 1959; Ford and Key 1956; Friedenberg 1957; Siffert 1956; Barash and Siffert 1966; Nordentoft 1969). At the end of the 20th century, first experiments were conducted with biodegradable materials. Mäkelä et al. (1987) studied the reaction of growth plates to a polyglactin 91 rod that was placed in the central portion of distal femoral growth plate in rabbits. Six weeks after implantation, the distal femoral physis showed similar growth disturbance as a drill hole which was left empty. In a later study, the same author (Mäkelä et al. 1989) showed that the size of the implant played a significant role in the development of growth disturbances. In rabbits with transphyseally placed 2.0 mm polydioxanone (PDS) implants no permanent growth disturbances could be found, whereas 3.2 mm PDS implants caused a growth disturbance similar to a drill hole. In a large animal study on sheep, placing a biodegradable interference screw over the growth plate resulted in a deformity of the proximal tibial growth plate. The fact that no major growth changes were observed may have been related to the fact that the inteference screw was placed centrally in the growth plate of the proximal tibia, similar to Stadelmaier’s study (Stadelmaier et al. 1995). This revealed that microscopic growth plate deformities may also occur with the implantation of biodegradable materials and the authors did not recommend the use of such transphyseally placed implants in pediatric ACL reconstruction (Seil 2002).

In 1956, Ford and Key showed that a central lesion of the distal femoral growth plate in rabbits of a significant size could be responsible for a symmetric shortening. In one group of animals they made a simple drill hole whereas a complete destruction of the growth plate was performed in a second group of animals. The disturbance of longitudinal growth was maximal in group 2. In a third group of animals they destroyed the perichondral structures completely and found that these lesions induced severe axial deviations. In the same year Siffert (1956) drilled a 2.8 mm K-wire through the medial physis of the proximal tibia in rabbits or created an injury with a curette. He noted that that the central lesions caused decreased longitudinal growth whereas axial deformities could be seen in the group with the peripheral injuries. While these early studies documented a difference between peripheral and central physeal lesions in terms of axial malalignment, little is known about the regional differences of an injury within the physis. Since most of these experimental data resulted from small animal studies, it remains questionable whether their conclusions may be extrapolated to larger animals or even to the human knee. Nevertheless, animal experiments (Seil et al. 2008; Edwards et al. 2001) and clinical reports on complications (Koman and Sanders 1999; Robert and Casin 2010; Kocher et al. 2002) showed that the anatomical location of a peripheral growth plate lesion (medial, lateral, anterior or posterior) might have an impact on specific growth abnormalities.

The cut-off point from which a bone bridge cannot be spontaneously resorbed and from which the amount of injury or the cellular destruction may create growth abnormalities has not been completely evaluated yet (Pous et al. 1979; Bonnel et al. 1984; Seil 2002). A first attempt to quantify the critical size of the growth plate injury susceptible to create a growth abnormailty was undertaken by Johnson and Southwick (1960) in a rabbit model. They estimated it at 11% of the physeal surface. Nordentoft (1969) estimated the diameter of a bone tunnel to 20% of the width of the physis in the frontal plane which corresponded to approximately 10% of the physeal surface. Mäkelä et al. (1988) evaluated the size of a growth plate lesion of 2 respectively 3,2 mm bone tunnels at 13% (20%) of the width of the distal femoral physis in the frontal plane and 3% (7%) of the physeal surface. The larger diameter tunnels created growth abnormalities with significant shortening in 19 of 20 animals, whereas only minor shortening (<0,5 mm) occurred in 3 of 20 animals in the group with tunnel diameters of 2 mm. Janarv et al. (1998) confirmed these findings in another rabbit model and postulated that the relative size of a physeal drill injury necessary to cause growth disturbance starts at an injury of 7-9% of the physeal surface. Similarly, Garces et al. (1994) found no growth disturbances with surface lesions of 3,2% in the rat model.

In the human knee, Janarv et al. (1998) measured the surface of the tibial growth plate and estimated that a drill injury with a diameter of 8 mm corresponds to a lesion of 3-4% of the physeal surface. Using an MRI module for adolescent ACL reconstructive surgery (MAARS), Kercher et al. (2009) confirmed this finding in 10–15 years old children. Their module allowed for creating a 3-dimensional representation of the distal femoral and proximal tibial physis. They found that a variation of graft diameter from 6 mm to 11 mm increased the volume percent removed from 2.3% to 7.8%, corresponding to which approximately 1.1% for every 1 mm increase in tunnel diameter.

Little has been published so far on this specific subject. In contrast to the amount of injured central growth plate, the minimum size of a lesion of the perichondral structures causing a localized growth disturbance or even a complete growth arrest has not been completely investigated yet. In a previous animal study on 4 months old sheep, we obtained a severe malalignment of the distal femoral physis by drilling a 5 mm tunnel through the perichondral structures and by avoiding to fill the tunnel with a tendon graft. An MRI analysis of the injured bones at 10 months revealed that the injured size of the perichondral structures was 3-5% of the size of the entire perichondral circumference (Seil 2002). Further studies need to confirm these data and to evaluate more precisely at which time point of the growth period and from which amount a localized peripheral growth plate injury may become deleterious for further longitudinal growth.

If the growth plate is perforated perpendicularly during tunnel drilling, the drill injury is round and its size can be deducted from the following formula (Figure 5a and b):

A=πxr2,wherer=a/2.

In the following this theoretical example will be transposed to the femoral growth plate for either transphyseal or anteromedial portal drilling: The angle between the inclination of the drill and the femoral tunnel with transtibial drilling is approximately 25°, whereas it increases to 75° with drilling through the anteromedial portal. Considering a tunnel diameter of 7 mm, the former will result in a growth plate injury of 42 mm², whereas it will result in a 3-fold increase of 148 mm² with the latter. Increasing the tunnel diameter to 9 mm will result in a growth plate injury of 70 or 245 mm² respectively. In terms of volume of the growth plate Kercher et al. showed in a specific MRI module that increasing the tunnel drill angle from 45 degrees to 70 degrees decreased the volume percent removed from 4.1% to 3.1%, which averages 0.2% removed for each 5 degrees increase in drill angle. These examples show that both inclination and tunnel diameter are critical.

The probability of a bone bridge development is supposed to be higher in older children in whom the activity of the growth plate has already decreased. It is well known that severe growth arrests may be transitional in younger children in whom the large remaining growth potential may counteract a strong localized bone bridge. In these cases, the growth plate is able to develop a sufficient force to break a bone bridge early on (Ford and Key 1956; Friedenberg1957; Campbell et al. 1959; Nordentoft 1969; Pous et al. 1979). A similar process can be deducted from animal experiments performed by Barash and Siffert (1966)). They splitted the distal radial epiphysis cats and dogs and interposed a synthetic membrane. At the time of operation the animals were aged from several days to 8 ½ weeks. The trend to develop a bone bridge was highest in those animals with the largest epiphyseal bone core and hence the smallest distance between this epiphyseal bone core and the metaphyseal bone (Figure 6). In these animals a dip deformity of the growth plate could be documented. 5.

As a consequence of these findings one could assume that due to the low remaining growth potential in older patients with little growth remaining, the risk of a significant growth deformity is also reduced in these patients. In fact, adolescents are at a higher risk of epiphysiodesis but with little clinical consequences in terms of growth disturbances, whereas young children are at a lower risk of epiphysiodesis but sometimes with dramatic clinical consequences if the physeal bridge persists and continues to develop until the end of growth. Pediatric ACL reconstructions, and especially the transphyseal technique, in adolescents close to knee skeletal maturity may not be a benign procedure and should be performed with great care in this period.

Friedenberg (1957) resected the periphery of the growth plate and its neighboring bone. He noted not only the formation of bone bridges over the resected area but also a frequent resumption of longitudinal growth. This indicated that the bone bridge may be resorbed by the distraction forces induced by the cartilage of the growth plate. An attempt to quantify this distraction force was made by Strobino et al. (1952). They found that the force which was necessary to cause a growth arrest of the bovine proximal tibial growth plate approximated 11,5 to 19,2 mg/mm². In rabbits this amount seems to be much higher (15 g/mm²) (Bonnel et al.1984). By analyzing the resistance of bone cement which was used for tumor prosthesis in 7–13 years old children, Safran et al. (1992) estimated the force induced by the growth plate to 58 kg per square centimeter.

Ono et al. (1998) performed ACL reconstructions with iliotibial tract tendons in 25 rabbits and with patellar tendons in 31 rabbits. Despite the fact that most of the grafts were ruptured, significant growth abnormalities with limb shortening and axial deformities could be found in 10 animals in which the grafts remained intact. It was concluded that these abnormalities were secondary to the high tensioning of the grafts. The authors created the term tenoepiphysiodesis which describes the fact that an excessively tensioned graft which is thoroughly fixed both proximally and distally may be able to create compression forces on the growth plate and slow down the growth process according to the HUETER-VOLKMANN-principle (Hueter (1862); Volkmann (1862); Volkmann (1869)). This principle showed that an increasing pressure on the physis might inhibit longitudinal growth and vice versa. A similar effect could be found by Edwards et al. (2001) who performed ACL reconstructions with autologous fascia lata grafts on twelve 8 weeks old dogs. The authors tensioned their fascia lata autografts to 80 N and found a significant femoral valgus deformity. In these animals limp shortening could be found on the lateral femoral and the medial tibial side. Even if the ideal tension of ACL grafts is unknown to date, it is reasonable to believe that a tension of 80 N is excessive in skeletally immature patients. In our own findings we could show that an intraoperative tensioning of Achilles tendon autografts to 40 N did not result in major growth disturbances (Seil et al. 2008). Similar clinical complications in pediatric patients could be described by Kocher et al. (2002) and Robert and Casin (2010)). Both found a valgus deformity after ACL reconstruction in children. Similarly, placing a transphyseal synthetic graft material may lead to the same type of complication (Kurosaka 2013).

Following the description of the HUETER-VOLKMANN principle, numerous clinical examples which were reinforced by further experimental work among else by Bonnel et al. (1984), showed a rebound effect of longitudinal bone growth. This means that a transitional stimulation of longitudinal growth may occur after the removal of a supra physiological constraint. Chudik et al. (2007) found a significant increase in longitudinal growth compared to the control side in the tibia in transphyseal ACL reconstructions in dogs. This longitudinal overgrowth process can also be observed on a regular basis in children, be it in association with tibial fractures or with pediatric ACL reconstructions and may be transient (Chotel et al. 2010). So far it is not clear to which extent rebound and overgrowth effects are related to each other.

Chudik et al. (2007) studied the reaction of growth in rapidly growing 10-week-old dogs with three different variations of femoral drill holes. They used a central tibial tunnel and three variations of femoral graft placement: epiphyseal, over the top, or transphyseal. All techniques had a high rate of graft failure and none of them was shown to be superior. There were no significant differences in femoral longitudinal growth. The epiphyseal technique reproduced the anatomic position of the ACL graft most closely but resulted in the development of rotational deformities, a growth abnormality that had not been reported before. MRI revealed more chondral and subchondral damage on the lateral femoral condyle with the epiphyseal technique. Further investigation is needed to understand this observation and its clinical implications. So far, this deformity has not been described in the human knee.

There are several experimental studies analyzing the graft incorporation in animals with open growth plates. Meller et al. (2008a,b, 2009a,b) found that in immature sheep Sharpey-like fibers develop after 3 weeks post operatively at the passing site of the graft. The maximum load to failure was decreased to 37.8 +/− 17.8 N after 3 weeks, but restored to 522.9+/− 113 N after 24 weeks. At 24 weeks, the knee had regained 69% of its biomechanical strength. The authors could demonstrate that the remodeling of the soft tissue graft is faster and more complete in immature sheep than in mature sheep. Further biological investigations were performed by Magarian et al. (2011) who studied the age related function of ACL-fibroblasts. They could prove that cellular migration and proliferation of human ACL-fibroblasts is age determined in a similar way as described in large animal models. The findings indicate that immature patients are more likely to benefit from tissue engineering than adolescent or adults. This presumption needs to be still verified in future studies. Murray et al. (2010) did their research on minipigs that concluded the better and faster healing of soft tissue in immature individuals. The research group from Murray (Murray 2009, Fleming et al. 2009) has done its work over better understanding the ACL healing in biological-cellular level. They could show promising results in minipigs after using Collagen-platelet composites around the standard ACL graft. After a dissected ACL was sutured and grafted with CPC, the control group was treated with suture alone. Load at yield, maximum load and ACL tangent modulus were all significantly higher in the suture repairs augmented with collagen-platelet composite than in repairs performed with suture alone. This work confirms the bigger biological potential of pediatric soft-tissues as compared to adults. However, its clinical relevance in the field of pediatric ACL reconstruction is still under investigation (Proffen et al. 2013).