The Anterior Cruciate Ligament (ACL) is one of two central ligament stays in the centre of the knee joint, helping to secure the relationship of the femur to the tibia. The two ligaments can be seen within the notch between the rounded ends of the femur when the knee is bent. In small children the ACL is 'weedy' and not the same strong structure that you see in the adult.

The ACL prevents the tibia sliding forwards in relation to the femur when a person stops suddenly, and helps to contain rotational movement between the two bones.

ACL injuries in children are increasingly becoming the remit of the orthopaedic ligament surgeon. The paediatric ACL injury has traditionally been thought of as relatively uncommon but with the increase in competitive sports, better awareness, improved imaging and greater demands on athletes at an earlier age, we are seeing a significant rise in the number of children coming through - particularly in their adolescent years - with ruptured anterior cruciate ligaments. 

Australian Medicare statistics in patients under the age of 16 show the numbers of reported ACL ruptures increasing year on year through to 2010, and this is certainly a trend that is being seen throughout the western world.​

When it comes to how these injuries should be managed in this group, the 'growth plates' or 'physes' of children dominate the debate. The discussions focus upon whether or not you should do nothing at all surgical - just go down the conservative route - or whether you should operate and risk damaging the growth plates. Parents and doctors have to make difficult decisions on behalf of these young people, and some are only small children not yet in their teens who cannot fully participate in the decision-making process.

If a choice is made to NOT operate, there are some real issues with compliance with bracing with small children, and these young patients may go on to develop chronic knee instability. Instability can interfere not only with sport but also day to day activities, and leave patients coping poorly. Episodes of the knee 'giving way' may result in further secondary injury of the meniscus or joint surface, and this can be disastrous.

It is now well appreciated that such conservative management gives poor results, but the concern continues that surgical efforts at re-stabilising the knee could jeopardise the growth plates, resulting eventually in a shortened or deformed limb.

Showing the percentage contribution of the various growth plates to the eventual length of the long bone

Long bones in the growing child get longer, not from the ends of the bones but from an active cartilage 'growth plate'. This is properly called a physis, and is sandwiched between the end of the bone and the main body of the bone. On X-ray the physes look like fractures, but that is because the X-rays pass easily through cartilage and less easily through the bone. The end of the bone to the one side of the physis is called the 'epiphysis' and the shaft of the bone to the other side is called the 'metaphysis'. So the physis is like a cartilage 'sandwich' within the bone. As new cartilage is formed it adds to the length of the metaphysis and this subsequently matures into new bone.

The knee bones from the side, showing the growth plates or physes

The various growth plates contribute different amounts to the final length of the adult bone. At the knee the growth plates of the thighbone or femur contribute to 70% of the eventual length of the thigh, while the growth plate of the shin bone or tibia contributes 55% of the eventual length of the shin. You can appreciate that, although both are important, it is particularly critical to try not to damage the growth p​late of the femur.

Injury to the physis may result in a slowing down or even arrest of growth in the limb, or an angular deformity.

Children who injure their ACL tend to play contact sports like basketball, or 'cutting' sports like soccer, where there are sudden abrupt turns and stops, and sudden changes of direction. The injury might also occur on jumping and landing on the feet on a hard surface, when the immature quadriceps muscle of the lap is not strong enough to contain the action.

Teenage girls are especially at risk, with ACL injuries several times more common than in boys of the same age, because they have increased risk factors to do with their body shape, their limb alignment, their muscle bulk and hormonal influences.

It may be difficult to obtain the history from the child, but the parent may have been an observer or spoken to witnesses, so it is always important to ask the parent, too. The child may have felt a 'pop', and been unable to bear weight immediately afterwards. Sudden onset of marked swelling may be reported. Typically things may then seem to settle down, but the child may have an unusual gait or be reluctant to participate in activities.

The ACL damage may be proximal (at the attachment to the femur), mid-substance within the ligament, or distal (at the attachment to the tibia).

Generally the orthopaedic surgeon sees the child after the swelling has reduced and the parents are wondering if there is actually anything wrong with the knee. Examination, however, typically shows a lax knee, with positive signs on performing Lachman, anterior drawer and pivot shift  tests.

Note that in children, the degree of translation for such laxity tests - for both operated and normal knees - is generally higher than that documented for adults. This may be due to constitutional increased joint mobility present in children.

On an MRI scan the normally 'weedy' ACL in a child may look thickened when torn because of oedema (or fluid in the tissues), and this may confuse the observer. Signs of bone oedema on tibia and femur may hint at the damage.

Bernard Moyen and his team [1] looked at rupture of the anterior cruciate ligament in children and they took 2 groups - those that they operated on quickly or those upon whom surgery was delayed until skeletal maturity. In 56 patients where the mean times to surgery was less than 5 months in the one group and more than 30 months in the other, the number of meniscal tears went up from 14% to 41% with the delayed group.

Another paper by Justin Roe's team [2] was a meta-analysis of 5,086 patients. They found that there was two times the risk of secondary damage if there was a five month delay to surgery, and six times the risk if there was a one year delay, and double the risk in anyone under the age of 17.

A further paper by Todd Lawrence [3], looking at 14 year data in patients where there was a delay of more than 12 months, again showed a massive increase; a four-fold increase in new meniscal tears, 14% of which were non-repairable bucket handle tears, and a significant number of lateral condyle injuries.

So should we wait? Probably no, and that is the feeling of many surgeons who undertake the treatment of this injury in children.

So far we considered the option of not operating until the growth plates have closed. The evidence for following a conservative route was weak. Many surgeons would agree that the potential for years of instability, with a high probability of secondary damage to the internal structures, supports a decision to consider surgery. So in Part 2 we will look at the issues surrounding surgery for ACL injury in children.

The main concern about surgery is growth disturbance and this is a really important complication - but it’s extremely rare and certainly a lot more manageable if it does occur than if we see secondary damage to the joint surface or the meniscus in a developing adolescent.

So the consensus and the evidence is to treat, but there is no consensus over how we should carry out the surgery:

The potential growth disturbance on the femur is angular deformity - usually valgus - meaning that the knee can assume a 'knock-knee' appearance. If any growth disturbance is seen on the tibia, it is usually in the form of leg-length discrepancy or the leg bending backwards - what we call 'recurvatum'.

There are also some issues about what grafts we should use in children, particularly small children.

We use a scoring system called the Tanner Scale to help with decisions about how to tackle the surgery. The Tanner Scale is based mainly around the secondary sexual characteristics of puberty. For the very young - boys of less than 14 and girls less than 13 who have not entered puberty - it is recommended to use a tunnel that does not go through the growth plate on the femur, but to stay below it.

Let's look at the options for graft and tunnel placement in a bit more detail:

1. Extra-articular graft placement

The strip of ITB is fed over-the-top, back into the notch, and under the intermeniscal ligament, to be fixed by a screw on the tibia below the growth plate.

The first option is to try to stabilise the knee without going through the joint cavity or the growth plates - what we call an ‘extra-articular’ approach’. Early work at extra-articular stabilisation focused on techniques where tissue could be used in the so-called 'over-the-top' position. In these techniques there was no tunnel in the femur whatsoever.

A strip of the iliotibial band (ITB) is taken from the side of the thigh, preserving its lower attachment on the tibia. This strip of strong material is fed around the rear of the knee and back to the front of the joint without putting a tunnel through either femur or tibia. The free end of the ITB is secured below the growth plate on the tibia, so the growth plates are totally avoided.

Papers have been published to show good results, but this is not a technique that is currently undertaken by many surgeons.

2. Trans-physeal graft placement

The next option is that of drilling right through the growth plates - what we call the ‘trans-physeal’ option. The vast majority of angular deformities and growth disturbances after ACL surgery in children have been associated with bone plugs or fixation devices deployed across the growth plate. It is also considered that drilling may cause heat damage and implants may have a pressure effect. Despite these risks, trans-physeal ACL reconstruction is frequently performed in skeletally immature patients with good outcomes, no or minimal growth disturbance and a high rate of return to previous activity levels. Tunnel positions generally need to be compromised, though, because although oblique tunnels are biomechanically favourable, they effectively increase the cross-sectional area of growth plate disruption.

3. All-epiphyseal graft placement

The illustration is demonstrating 'tunnels' but a less invasive option is to use 'sockets', where there is a narrow portion that accommodates a suspensory thread and a wider portion that accommodates the ligament. That will be discussed in more detail in the second section.

One of the real innovations in ACL surgery in general has been the development of what we call the 'all-inside' technique. This involves drilling the bone tunnels (or the newer 'sockets') from within the joint cavity, using 'retro-drills' such as the 'FlipCutter'. With this approach and the associated instrumentation, guided if necessary by fluoroscopic imaging, surgeons can go 'all-epiphyseal' - staying inside the ends of the bones without penetrating the growth plate.

In this option, the surgeon keeps the tunnels or sockets within the ends of the bone - that is within the epiphysis - and does not go anywhere near the growth plate. Gauges and fluoroscopic imaging ensure that the surgeon hits the target and does not compromise the growth plates.

4. Hybrid graft placement

If necessary the surgeon can perform a hybrid all-inside procedure, staying under the growth plate in the femur, but creating a tiny 3.5 mm drill hole through the growth plate on the tibia, using it to retro-drill from inside the joint a short but wider tunnel big enough to accommodate the graft.

As well as options for the placement of the tunnels, there are options for the kind of graft tissue or other material used for the stabilisation procedure.

Timing of the surgical intervention in the child is critical. Early ACL reconstruction in children (within 6 weeks) is associated with improved results and threefold fewer medial meniscal tears [4], whereas delayed operative intervention leads to higher rates of meniscectomy and lower subjective outcome scores [5].

However, the high re-rupture rate in this population group continues to be a factor, and we hope you will find the ideas offered in the remainder of this course relevant in this respect.

With the patient asleep and relaxed, the pivot shift, Lachman and anterior drawer can be subjectively repeated and the laxity clinically confirmed and compared with the normal side.

Attempts may be made also to objectively record the laxity. The KT-2000 is just too bulky for the knee in small children. A better option is the KIRA (below) - which is app-based.

Prior to any definitive decisions regarding the surgery, an arthroscopy is performed through anteromedial (AM) and anterolateral (AL) portals. The surgeon assesses:

Patient 'R'

This young girl suffered a knee injury at the age of 5 while jumping on a trampoline. Unseen by her, a friend threw a hard ball onto the trampoline, and the young patient inadvertently landed from a height onto the ball with her knee bent.

If you have not yet watched the video interview with the parent, scroll up to the top of the page and you will hear the story there. What is interesting is that the initial excruciating pain settled after a few days, and the parents only continued to be concerned because although she was getting around she was doing so with a rather strange gait.

The growth plates are quite clearly seen on the MRI, and the area where one would expect a clear outline of the ACL is indistinct, suggesting damage to the ACL.

Examination-under-Anaesthesia

In this clip the surgeon examines the knee for laxity while the young patient is under anaesthesis and fully relaxed.

A point to note is that pivot shift and anteroposterior translation are higher (for operated and normal knees) in children than adults, but this may be due to constitutional increased joint mobility present in children.

Arthroscopic Examination

Arthroscopy was performed via standard portals, revealing that the ACL had torn away from its femoral origin, but that the body of the ligament was intact - a perfect situation for a repair.

There was also a partial tear of the meniscus. You can see this as a very feint white line on the top of the meniscus.

The tear did not actually require any attention.

The empty notch at top end should have been filled up with ACL where it normally originates on the femur, but the ligament has come away from the bone here and left a gap.

The ACL itself is otherwise intact.

The ACL Repair Procedure

The first step of the procedure is to lasso the free end of the ACL, so that the surgeon can pull it out of the way to prepare the repair site, and then eventually pull it right back into its original position and fix it there. This lassoing is performed via the arthroscopy portals using an instrument called a 'mini-scorpion'. 

A small incision is made through the skin on the lateral side of the femur, and gauges and fluoroscopic imaging are used, to carefully drill a 2.4mm drill hole through the epiphysis, taking care to stay under the growth plate:

A double-stranded 'passing suture' is passed through the femoral tunnel. This will eventually be used to pass the FibreTape internal brace through both femur and tibia. A second 'repair suture' is passed through to hook the lasso and tension the ligament.

The lasso (on the left of the video) is pulled up through the femoral tunnel and the ACL is re-approximated on the side wall. The FibreTape for the internal brace is ready on the right, waiting for the drilling of the tibial tunnel.

Because the surgeon elected to do a hybrid approach for the internal brace, that is avoiding the growth plate on the femur but passing through it on the tibia, the tibial tunnel - again 2.4 mm - was drilled through the growth plate. The sutures holding the remnant are tied onto the suspensory fixation tightrope button which sits against the lateral femoral cortex. The FibreTape is secured and tensioned at the tibial end with a 4.75mm SwiveLock anchor​, with a button at the femoral end.

Second-look arthroscopy at 3 months

Three months later - immediately pre-op for the second-look arthroscopy, the patient demonstrated how well she was running around and pain free

During the arthroscopy the hardware was removed fairly easily, and with a clip on the FibreTape the internal brace just slid out really very easily. The rationale for removing the polyethylene braid was so that it would not tether the growth plate.

Examination of the joint was completely normal. There was no synovitis, the joint surfaces appeared normal, menisci were intact and the ACL healed. The ligament at the femur had really incorporated nicely, and was synovialised. It was not over tensioned..

Follow-up at 2 years

At two years the child was assessed for range-of-motion, limb length and ligament laxity. All of these were normal.

This is the device that we found useful to check the laxity in this small knee - the KIRA. This recorded a less than 2mm side to side difference.

Patient: 'Sh'

History

At the age of 6 this young patient had a skiing accident. He was in ski school, so the parents did not actually see the injury but the child explained that the ski binding did not release, and the torque of the ski severely twisted his left knee.

On fetching him, the mother was sure that he had torn his ACL, based upon her own experience as she had previously suffered an ACL tear herself. What is of interest is that by the time he went for MRI imaging two weeks later he was walking normally, but the MRI confirmed that the ACL was indeed torn.

After discussion with several clinicians, he was referred to Professor Adrian Wilson, who decided to repair the ligament and use an internal brace until it healed. He decided on an 'all-epiphyseal' approach.

Examination-under-anaesthesia and Surgery

Examination under anaesthesia confirmed that there was excessive antero-posterior laxity and increased pivot shift, confirming the damage to the ACL.

The surgeon continued to surgery, preparing to do an arthroscopy to assess the damage, after which is was decided to do a repair - all-epiphyseal​ - and to brace the repair with FibreTape.

The arrow points to the 2.4 mm drill within the epiphysis on the femur, that is below the growth plate of the femur. 

Similarly the tibial drill hole was kept within the epiphysis on the tibia (see arrow), that is above the growth plate.

One-year Follow-up

His toes are not quite level here in the right hand picture, giving the appearance of the left being shorter, but long leg X-rays confirmed that there was no shortening.

At one year he reported a normal knee with no subjective symptoms.  He wanted to play football but his mother was worried about this.

Clinical examination was normal, with negative Lachman, pivot shift and anterior drawer tests.​ There was no growth disturbance evident on long leg X-ray. Range-of-motion was -5 to +145 degrees. Scoring tests revealed a KOOS of 100, a Lysholm of 100 and a Tegner of 4.

The Lachmann test at 30 degrees & the pivot shift test assessed with a KIRA demonstrated less than 2 mm side-to-side difference​.

Commentary

Paediatric ACL injury is traditionally perceived as relatively uncommon. However, it is increasingly common in competitive sports, accounting for 0.5-3% of all ACL injuries. This may be because of:

• increased awareness
• improved imaging
• greater demands on athletes at an earlier age

This is a common type of injury seen in children, and is in fact the most common way in which rupture presents in the skeletally immature.

We have a classification - the McKiever classification - based on the amount of displacement, small, moderate or severe amount of displacement.

Patient: 'O'

This seven year old girl was referred to Professor Wilson from another hospital and presented with a nasty injury to her right knee.

She had twisted the knee whilst playing football and rather than rupture the ligament she pulled off the attachment of the ligament where it inserts onto the tibia. This can be seen on both of the X-rays shown below. 

X-rays

The arrow marks out the site on the lateral view on the left the bone fragment which is sitting up and represents really quite a large part of the central aspect of the upper part of the tibia. We can see the bone fragment again on The AP or straight view, which is on the right.

The lateral view of the knee

The A-P (antero-posterior) view of the knee

Fracture reduction & fixation

The technique used for fixing this fragment down was to create two small drill holes from the front, and come up and pass sutures over the top of that bone fragment which he reduced via keyhole surgery, and effectively lashed that piece of bone back down into position.

Normally we would then take patients very slowly through their rehabilitation, but in this case Prof Wilson also drilled up through the middle and across the ACL to an attachment point on the femur. He then created a tunnel through to the outer aspect of the femur and ran some FibreTape - the so-called internal brace - which he fixed with a button on the side of the femur and with a little screw on the front of the shin bone...without any immobilisation or need for a brace.

Arthroscopic findings

The video on the right shows the arthroscopy at three months when the surgeon did a second look, and removed the hardware. The video might be a little confusing, as starts off by showing the patellofemoral joint (0:08) and then the camera moves to the rounded end of the femur (0:13), and continues down into the medial side of the joint. where we can see the ACL nicely reduced (0:17).

Follow-up at 4 months

Olivia was then followed up at four months, where you can see that she was comfortable with a stable knee. The anterior drawer at 90 degrees was beautifully stable with an excellent range of motion. 

Follow-up at one year

At one year follow-up, you can see that she has full range-of-motion.

The one-year X-rays show how nicely that bone fragment has been reduced and held in position.

Of course, Olivia is being followed up long term until skeletal maturity to make sure there is no growth disturbance but the risk of this is really minimal. She is now back to full activity with no limitations.

This is an excellent way of treating this injury. The historical techniques of just allowing these bony avulsion injuries to heal in a long-leg plaster is really totally inadequate. We should go for anatomic reduction, strong good fixation and early mobilisation - a mantra for any injury within the joint.

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