Introduction
Osteochondrosis dissecans (OCD) is a well-known orthopedic developmental disorder occurring mainly in the fetlock, hock and stifle joints of young horses.
It is generally accepted that arthroscopic treatment at a young age (1.5-2 years old) results in a good athletic prognosis. The classic arthroscopic treatment consists of removing the fragments and debriding the OCD bed where needed to healthy subchondral bone.
For the femoropatellar (FP) joint, the long-term results and recovery as a high-level sport horse might be more disappointing when dealing with large osteochondral lesions. The outcome depends on the age at which the horse is operated but also on the extensiveness of the OCD lesion and the experience of the surgeon (4,5,7).
In potential future high-level athletes preserving as much hyaline cartilage as possible is mandatory and this can be done by re-attaching the osteochondral flap instead of removing them from the FP joint. Dr Alan Nixon and his co-workers at Cornell university were the inventors of this technique and they published their results on re-attachment of osteochondritis dissecans cartilage flaps using resorbable polydioxanone pins in 2004 and 2011 [2, 3]. Our group at the Bosdreef Equine Hospital has continued this work and in 2019 we started using Conmed SmartNails to re-attach the osteochondral flaps.
From 2019-2023, 81 FP joints were treated in 59 warmblood foals with SmartNail fixation.
Radiographic resolution of the OCD lesion was found in 64 (76 %) , partial healing in 10 (12%) and no healing in 10 (12%) of the lesions.
The patient selection, surgical technique and results will be discussed during this lecture.
Patellofemoral pain syndrome:
OCD of the FP joint often leads to an additional problem compared to fetlock and hock OCD and the most important actor here is the patella.
The patella is often forgotten when dealing with FP joint OCD, it is only a small piece of bone located between the quadriceps and patella ligaments, but the patella is as important to the FP joint as the menisci and cruciate ligaments to the FT joints. The patella is the pivot point, the fulcrum that delivers extra leverage to the quadriceps muscles and is very important during high physical activities. It has a close and tight contact with the femur and is firmly held in place by 3 patella ligaments and a 2 femoropatellar ligament. The lateral articular facet of the patella will exert its maximum pressure on the lateral trochlear ridge of the femur at the level of its proximal to middle third with the FP joint in semiflexion. This is precisely where we usually see the OCD lesions and the cartilage damage to the patella.
The constant frictional and pressure contact between the lateral articular facet of the patella and the OCD lesion, not only prevents spontaneous healing of the OCD lesion but may cause loosening of the osteochondral flap and damage to the articular surface of the patella leading to chondromalacia of the patella. On top of this, the tight contact between femur, patella and joint capsule may also lead to damage of the joint capsule opposite the OCD lesion.
The patellofemoral pain syndrome is caused by a combination of subchondral bone pain and capsule pain:
1° The patella: The chondromalacia of the patella, the weakening, fissuring and erosion of the cartilage makes the underlying bone pressure sensitive.
2° The lateral femoral trochlear ridge: The operated OCD lesion will be covered by biomechanically inferior fibrocartilage which cannot withstand the patella pressure at high physical activities also leading to a pressure sensitive subchondral bone.
3° The joint capsule: The damage to the FP joint capsule can lead to synovial pain and capsule pain caused by exposed nerves.
Conclusion:
Sport horses operated from FP joint OCD may have difficulties in maintaining soundness at high level of physical activities because of this patellofemoral pain syndrome. This becomes more important at an older age (more wear and tear) when the horse is competing at higher level (> 1m40 jumping, higher pressure between patella and femur). These horses are often not lame, but riders are complaining about “loss of strength” in the stifles and loss of performance.
To prevent this patellofemoral pain syndrome for aspiring top level athletes it is crucial to preserve as much as hyaline cartilage as possible.
The goals are:
1° operate before there is damage to the lateral articular surface of the patella
2° restore the normal contour of the lateral femoral ridge with solid subchondral bone.
3° maintain hyaline cartilage over the lateral femoral ridge.
Implant presentation:
In the original technique described by Dr Alan Nixon the osteochondral flaps were re-attached using PDS pins. Those pins are no longer available.
We are currently using the 25 mm long, 1.5 m diameter Conmed SmartNails® to re-attach the osteochondral flaps in the FP joint.
The advantage of this implant is that the nail has a head which allows us to compress the OCD flap against the subchondral bone of the femur. Unlike OCD fragments in humans, where a large chunk of bone is present underneath the flap, this is not the case with OCD in the horse. Only a very thin layer of bone is present underneath the osteochondral flap and that layer is too thin to obtain sufficient compression when using headless screws.
Surgical technique
Surgery is performed in dorsal recumbency through a standard sub-patellar arthroscopy portal. First a diagnostic arthroscopy is performed to assess the state of the osteochondral flap. The ideal osteochondral flap is all-round attached to the normal surrounding cartilage with no cracks and minimal elevation (< 5 mm) from the subchondral bone. The ideal flap to re-attach is covered with smooth, non-fibrillated or fissured cartilage. The older the lesion, the more the flap is elevated, and the less healthy blood supply and healthy bone cells are present. The working portal is made in relation to the lesion location but is in general located cranial to the lateral ridge and just distal to the lateral base of the patella. In most cases only 1 working portal is needed to place all the screws no matter the length of the lesion. Drilling and placement of the SmartNails is performed through a custom-made guide. The head of the nail will compress the osteochondral flap to the subchondral bone. In general, 1 nail is placed per ± 1.5 cm2.
In older lesions a part of the flap might be disrupted from the neighboring normal cartilage with no viable subchondral bone underneath. In that case a partial re-attachment of the viable part can be combined with a removal of the non-viable part.
A power osteostixis with a 1.2 m K wire is then performed at several places through the flap to promote inflow of blood from the subchondral bone and promote healing. Exceptionally (in 10% of cases), a 2nd work port needs to be made to have proper access to the entire flap.
Post-operative care
Horses are box rested the first 2 weeks followed by 2 weeks box rest and controlled movement. After this they are allowed in a small field for 2 months. Three months after surgery they can return to a larger field together with other horses.
Evaluation of healing
Owners were asked to have follow up radiographs taken at 3, 6, 9 and 12 months after surgery to evaluate healing. Complete healing is defined as restoration of the normal contour of the femoral trochlear ridge with normal structure of the subchondral bone, absence of fragments or osteochondral flaps, no effusion of the FP joint and no lameness.
In these patients where there is no or only partial healing of the lesion, a classic arthroscopy with removal of the non-healed fragments is performed.
Data and Results:
Full data will be presented during the lecture
59 horses – M 29 (49%) F 30 (51%)
FP joints: 81 - LH: 45 (56%) RH: 36 (44 %)
Mean age at time of surgery: 8.5 months STD 1.93
OCD diagnosed by radiographic screening: 22 (37%)
OCD diagnosed because off clinical symptoms followed by radiographic screening: 37 (63%)
Radiography:
Length of OCD lesion was dived in 4 cathogeries:
A: < 2 cm: 26 (31%), B: 2-4 cm: 42 (51%), C: 4-6 cm: 11 (13%), D > 6 cm: 4 (5%)
Mean size of the OCD lesion in all FP joints: 2.9 cm - STD 1.26 cm
Mineralization of the osteochondral flap: 43 (52%)
Non visible osteochondral flap: 40 (48%)
Surgical findings:
Presentation of the OCD flap:
OCD flap completely attached to surrounding normal cartilage: 70 (86%)
OCD flap partially detached/torn from surrounding normal cartilage: 11 (14%)
OCD flap is covered with smooth cartilage : 72 (89 %)
OCD flap is covered with fibrillated / cracked cartilage: 9 (11%)
Elevation of the OCD flap from subchondral bone:
less than 5 mm: 41 (51%)
5 mm or more: 40 (49%)
Patella:
Patella cartilage intact: 72 (89%), not intact: 9 (11%)
Macroscopic view of the synovial fluid:
- Synovial fluid normal: 73 (90%)
- Dark yellow/blood stained/fibrine: 8 (10%)
Number off nails used per joint: mean 6.2 STD 2.3
Evaluation of the results after healing:
Radiographic results after healing in 59 horses/ 81 FP joints / 84 lesions :
Radiographic resolution of the OCD lesion: 64 (76 %)
Partial healing more than 50% of the lesion: 6 (7 %)
Partial healing less than 50% of the lesion: 4 (5 %)
No healing / failure: 10 (12 %)
Healing time:
20 (25%) healed < 8 months after surgery
61 (75 %) 8-20 months, most of them healed 12 months after surgery
Lameness after healing: most of the horses not or only just in work
- not lame : 80 (99 %) / non of the non-healed was lame.
- lame : 1 (1 %) case of partial healing.
Revision surgery:
20 (24%) FP joints non/partial healing: 18 revision surgery
Patients’ selection
To have a successful re-attachment of an osteochondral flap we need a reasonably fresh OCD lesion that still has sufficient healthy and vascularized subchondral bone attached to the flap. If the subchondral bone attached to the flap has no adequate blood supply, the flap will not re-attach. Simply put, a chondral flap cannot be re-attached, an osteochondral flap can be re-attached providing there is still blood supply.
The older the OCD lesion gets, the looser the flap becomes, the less vascularized it gets and the less likely it is to achieve complete healing after fixation.
Osteochondrosis dissecans in the stifle joint has a very dynamic character and spontaneous healing can occur. Lesions of the lateral trochlear ridge of the femur develop mainly after the age of 3 months, peak at about 6 months and the majority of the lesions start resolving at the age of 8 months [1]. Lesion that are still present at the age of 12 months will most likely never resolve (unpublished personal data). It is our experience that, once the lesions are still radiographically present at the age of 9-10 months, full spontaneous healing is very rare in warmblood horses (6).
We also noticed that the odds of failure increases, when the re-attachment surgery is delayed until the osteochondral flap has lifted for > 5 mm from the subchondral bone.
We presume the lesion is then older, reducing the amount of healthy perfused subchondral bone underneath the flap.
Imaging-based screening
Ideally, we need to detect the lesion in its early stage because of the narrow window of opportunity to operate successfully (somewhere between the age of 6-11 months). This gives us the opportunity to follow up the lesion with radiography and ultrasound and to check if spontaneous healing is still possible and to operate before there is too much elevation of the osteochondral flap.
Therefore, we advise a radiographic screening of both stifles in foals at the age of 6.5-7.5 months.
If an OCD lesion is present, an ultrasound examination of the femoral trochlear ridges can be performed to determine if the cartilage is lifted from the subchondral bone and to what extent (8). This information is crucial to find out if we are dealing with and “older” OCD flap with good or reduced chance of healing after re-attachment or a “recent” OCD flap with possible chance of spontaneous healing. When in doubt, follow up radiographs and ultrasound should be undertaken every 3 weeks to assess the progression of healing.
Because the development of OCD is such a dynamic process between the age of 4-8 months of age, the age of the lesion is more important than the age of the foal. For example, an OCD lesion on the femoral ridge that appears at the age of 7 months can be successfully re-attached at the age of 9 or even 10 months. If the same lesion appears at 5 months and is operated at 9 months of age, healing after re-attachment is more likely to fail because the flap is “too old” to re-attach.
Final decision
The final decision, if the osteochondral flap is worth re-attaching, will always be taken during arthroscopic surgery.
Conclusion
Re-attachment of osteochondral flaps in the FP joint of the stifle in foals is possible but the success rate is highly dependent on correct patient selection. The aim of this surgery is to restore a normal contour of the femoral trochlear ridge covered with hyaline cartilage, so the horse does not suffer from loss of performance due to a “patellofemoral pain syndrome” at a later age with an increased intensive workload.
The dynamic character of OCD development in the stifle gives us only a small window of opportunities in which complete healing can be achieved. This window is determined by the age of the lesion and not the age of the foal and is somewhere between 6 and 11 months of age. Treatment within this window of opportunities requires radiographic and ultrasonographic screening of the stifles in foals at an age of ± 7 months. A radiographically normal stifle can be obtained in successful cases.
Fig 1: Conmed SmartNail used for re-attachment of osteochondral flaps in femoropatellar joint OCD in foals. The nail is 25 mm long with a 1.5 m diameter. The head of the nail will compress the osteochondral flap to the subchondral bone. This is important because in horses there is only a very thin layer of subchondral bone present underneath the OCD flap that needs to be compressed against the subchondral bone.
Fig 2: Arthroscopic image of a re-attached OCD flap using SmartNails. 4 cm long OCD lesion of the lateral femoral ridge in a 7-month-old foal re-attached with 8 SmartNails. Notice how the proximal and axial edge of the flap is torn off from the surrounding normal cartilage (green arrows). This is caused by friction of the patella.
Fig 3: Radiography of the right stifle before and 12 months after surgery.
References
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8. Hoey S, O'Sullivan J, Byrne J, Devine S, Toomey W, McAllister H, Skelly C. Ultrasound screening protocol for osteochondrosis at selected predilection sites in thoroughbred yearlings. Ir Vet J. 2022 Apr 27;75(1):8.