Dr Matthew BrickOrthopaedic Surgeon
Anterior CruciateLigament Injuries
The cruciate ligaments and thecollateral ligaments provide stabilityto the joint by holding the bonestogether. Of those ligaments, theanterior cruciate is important in that itholds the knee together duringtwisting type activities. Duringeveryday walking and in straight linerunning, the ACL is hardly used. Assoon as any twisting is performedhowever, this ligament is essential.Without it the knee twists further thanit is designed to, giving a feeling thatthe knee comes out of joint. It is thisfeeling of coming apart that givesrise to the instability or loss ofconfidence in the knee that is seenwhen the ACL is torn. As it is arotatory instability, it occurs whentwisting or sidestepping is attempted,or when uneven ground isencountered. If major, it may occurin everyday activities. In somehowever, it occurs only on thesporting field, where it is hard toconcentrate on protecting the kneeand where sudden twisting andturning occurs.
Meniscal Cartlilage and Articular(Lining) Cartilage:
Figure 1: The lateral and medialmenisci attach near the centre ofthe tibia.The menisci function as space fillersto spread the load between thesurfaces of the femur and tibia. Theends of these bones are not thesame shape and thus the menisciare needed to make up for thatincongruity. The end of the femur isround and the top of the tibia is flat.They primarily function somewhatlike shock absorbers but they alsohave a secondary role to enhancelubrication and nutrition of thearticular or lining cartilage. They aremade of springy cartilage, a little likeyour ears.
Figure 2: The menisci act likethese chocks to stabilize andsupport the round load (Femur)ona flat surface (Tibia)
Loss of meniscus (particularly thelateral one) leads to a poor spread ofweight across the joint surface. Thismeans that loads are taken oversmaller areas of the joint, and hence,pressures are higher, causingincreased rates of wear of the liningsurface. It also follows that the moremeniscus that is lost, the faster thatwear occurs.
The articular cartilage covers theends of he bones of the knee jointand allows for its smooth movement.It is a shiny, white, ultra low frictionmaterial, that acts as a bearingsurface for the joint. (it is easily seenon the end of uncooked lamb bonesetc). This articular cartilage is verydifferent from the meniscal cartilages(or menisci mentioned above) and isthe most delicate and irreplaceablestructure within the knee. Once thisgets damaged and wear starts tooccur, the knee can no longer bereturned to its normal state. Injury to
this lining can be treated bydebridement, a process of cleaningup: removing loose fragments andsmoothing the remaining damagedsurface. This removes all thefragments which may potentially fallinto the knee and in a number ofcases, it also helps to decrease pain.Despite this however, a permanentdefect remains which shows almostno attempt to repair itself. Once adefect exists in the smooth liningsurface, further wear occurs withtime. In essence, it is this damage tothe bearing surface of the knee thatstarts off the progressive processknown as osteo arthritis.
Microfracture is the most commonlyused cartilage-repair technique. Thebare bone at the base of theulcerated area is cleaned untilsmooth. A very small awl or pick isthen introduced and 2mm diameterholes are punched in the underlyingbone. This allows bone marrow cellsto escape and form a healing cellpopulation. The intended result is afibrocartilage patch. (fibrous cartilageor scar tissue cartilage) Althoughthis is not as durable as the cartilagewe are born with, it is better thanbare bone.
The other factors that affect wearrates are age and usage. The younghigh demand athlete puts his kneesthrough much more than theweekend recreational sportsman.For this reason, a lateralmenisectomy in a sixteen year old, isvirtually guaranteed to produce wear,that is sufficient to be noticeable onx-ray within ten years. On the otherhand, a medial meniscectomy in a35 year old, may show very little
change on x-ray for 20 years. In theyoung therefore, menisci should berepaired whenever possible and thisis particularly so in the case of thelateral meniscus which seems to bemore important that its medialcounterpart. Family history is alsoimportant as osteoarthritis has aninherited component. An athletewho has a strong family history ofosteoarthritis is especially at risk ifhe or she loses a meniscus.
Injuries to the anterior cruciateligament occur most often in athleticactivities (especially twisting andturning sports, such as football andnetball) but may be ruptured in workinjuries and non-athletic activities.The injury usually occurs withoutcontact and often is associated witha sudden change in direction (egside stepping) or a sudden change inspeed (a deceleration injury). It mayalso occur with the body falling overa fixed leg or with a hyper-extension(over straightening) injury to theknee.The athlete often describes theincident My knee went one way andmy body went the other.
Figure 3: The ACL is the greyligament visible in the centre ofthis 3D knee model.
When the injury occurs, theindividual will often hear a pop orsnap or experience the sensation oftearing inside the knee. The kneethen swells almost immediately,because of bleeding from vessels inthe torn ligament. Generally theinjured person has to be carried offthe field and finds that any attempt toweight bear is difficult because theknee feels extremely unstable. Theimmediate feeling of instability isdue, not only to the loss of theligament, but also to a loss of thenerve fibres within that ligament.These nerves provide a sense ofwhere the joint is in space which iscalled proprioception. Loss of thatsense causes a loss of the sensationof how bent the joint is, how fast it isbending and so on. Without thatknowledge there can be no accuratefeedback to the muscles that movethe knee or to the muscles thatprotect the knee; and hence, controlof the joint may be lost, leading to a
feeling of instability or loss ofconfidence.
With time the feeling ofproprioception improves. This isbecause the nerve fibres in the otherligaments attempt to make up for theloss of sensation from the nervefibres in the anterior cruciateligament. This situation is neverquite as good as the original but, ifthe demands on the knee are low, itmay be sufficient to get by.
After the Injury
What happens after injury to theanterior cruciate ligament is asudden loss of control of the knee,which gradually returns. In mostpeople, it takes about two months toreach a level where they can thinkabout playing sport again. Thosewho seem to get back to sportwithout surgery (about 30%) oftenonly have partial tears of theligament. Whilst the injured knee inthis group may be looser thannormal, it is thought that functionmay be satisfactory because someof the proprioceptive nerve fibresremain intact. These are thought toprovide enough feedback to themuscles around the knee, to enablethose muscles to compensatesomewhat for the partial loss of theligament.
Even for those with a completerupture, the feeling of stability doesgradually improve over a 2-3 monthperiod. If by that time however, fullconfidence in the knee has not beenrestored, then that knee will probablynever be able to perform a twisting,turning sport again without ACL
reconstruction. If a return to thosesports is made, then a repeat injuryis likely, due to the ongoinginstability. From then on, every timethat the knee gives way, moredamage is done. Sooner or later,that damage will include injury to thearticular lining cartilage, which isirreparable. This may herald an endto impact loading type activities andin essence, represents osteoarthritis,which will progressively worsen withtime. Because of this risk it is nowconsidered preferable to reconstructthe unstable knee early on, thushopefully, preventing recurrent injury.Our studies show that the athletewho undergoes reconstruction in thefirst 6 months before many instabilityepisodes does better long term thansomeone who has the operation 2-3years later after multiple episodes ofgiving way.
In general patients with ACL injuriesmay be put into one of three roughlyequal size groups:
The first group contains people whodo well and return to their sportwithout too much trouble. In thisgroup the re-injury rate is not all thathigh and, as suggested above, themajority in this group, have partialtears only. Essentially, the re-injuryrate, over a two year period, isthought to roughly equal thepercentage tear of the ligament. Itcould be said that a 30% tearprobably has a 30% chance of goingon to complete rupture within twoyears if normal sporting activity iscontinued.
The second group contains peoplewith a complete ACL tear who seem
to do well until they play ademanding sport. They may evendo well at training but, on taking tothe field, a re-injury soon occurs.This group otherwise copes well inday to day life and thus, onlyrequires surgery if a return to twistingtype sports is desired. Sports suchas netball and football demand goodACL function and for people wantingto play these sports, even at lowlevels, reconstruction of a completelytorn ligament is recommended.
The third group contains peoplewhose knees feel frankly unstable ineveryday life. This group all requiresurgery to give their knee a feeling ofstability. That stability then protectsagainst further injury and furtherdamage to the knee.
Who Warrants Surgery?
Overall, it may be seen that a largenumber of people who injure theiranterior cruciate ligament mighteventually benefit from surgery. Theexact number is uncertain butcurrently it is thought