DRAFT

The deep digital flexor tendon (DDFT) pulls upwards from its insertion site on the back of P3, with traction from wrapping around the navicular bone, and pushes the toe downwards.  The force on the toe is increased when the toe is long.  For active flexion of the DIP joint, the deep digital flexor muscle contracts, and this increases tension in the DDFT.  Flexion of the DDFT is opposed by contraction of the common digital extensor muscles and the common digital extensor tendon, which inserts on the extensor process of P3.  The DDFT lifts the palmar aspect of P3 and thereby moves the centre of pressure forwards (towards the toe) in relationship to the centre of rotation.  Palmar rotation of P3 and having chronically high heels may lead to shortening/contraction of the DDFT.  DDFT tension changes in horses with chronic laminitis, and pain from laminitis may also lead to changes in DDFT tension.  Prolonged tension on the DDFT may contribute to reducing lamellar perfusion and increase shear forces on the dorsal lamellae.  Ramsey et al. 2011 found, in an in vitro study, that elevating the heel up to 15 degrees might increase shearing forces on the dorsal lamellae.

A deep digital flexor tenotomy - cutting the deep digital flexor tendon (DDFT) - is sometimes recommended following laminitis when correct realigning trimming and support fails to realign the feet.

There are two recognized procedures - the DDFT can be cut either:
1.  at the mid-metacarpal (the middle of the back of the canon bone) - less risk of complications, but less release of tension in the tendon is achieved, and possible risk of metacarpophalangeal joint (canon bone to P1) contraction;
2.  at the palmar pastern (the back of the pastern) - increased risk of complications e.g. DIP joint subluxation, but also increased release of tension in the tendon.


DDF tenotomy is indicated in horses that have chronic phalangeal/bony rotation, i.e. a line through P2 and P3 is broken forwards, the distal interphalangeal (DIP) joint is abnormally flexed, and the palmar angle is higher than normal (>10 degrees), and correct realigning trimming to slowly lower the heels (without removing sole depth in the front of the foot if sole depth is insufficient) has been attempted but caused the horse to raise the heel  (In this case, before considering a DDF tenotomy, ask whether the heels could have been lowered too quickly, replace heel material lost with padding, and if the horse can be made comfortable, try again to very slowly lower the heels, possibly aiming for 5 or more trims to lower the heels, and possibly making the initial aim (at the end of the 5 or more trims) around 8-10 degrees rather than the usual 3-5 degrees following laminitis).  In some cases the fetlock joint may be abnormally flexed forwards with the horse walking on the toe of the foot, unable to lower the heels to the ground, even with abnormally high heels.

DDF tenotomy is not suitable for horses that have a negative palmar angle, or where lamellar connections in the back of the foot are compromised.  The back of the foot should be stable before considering a DDT tenotomy.

Complications of DDF tenotomy include:

• surgical errors damaging blood vessels,
• surgical errors damaging other tendons,
• post-surgery sepsis,
• making the back of the foot less stable,
• subluxation (partial dislocation) of the distal interphalangeal (DIP) joint (between P2 and P3) (more common following palmar pastern tenotomy),
• metacarpophalangeal joint contraction (more common following mid-metacarpal tenotomy).

Opinions vary on how to manage the foot following a DDF tenotomy, and depend on which tenotomy method was used.
The back of the pastern tenotomy gives a greater release of tension in the DDFT but also creates increased instability in the DIP joint.  Applying a shoe or device with a raised and possibly extended heel is recommended to prevent subluxation of the DIP joint.  
Some vets recommend a shoe or device with a raised and possibly extended heel following the mid-cannon tenotomy as well.
Pressure wraps over the canon bone and long-term box rest (up to 3 months) may decrease the amount of scar tissue formation and thickening of the DDFT and reduce the risk of metacarpophalangeal joint contraction.

References:

Ramsey GD, Hunter PJ, Nash MP
The effect of hoof angle variations on dorsal lamellar load in the equine hoof
Equine Vet J. 2011 Sep;43(5):536-42. doi: 10.1111/j.2042-3306.2010.00319.x
"The models in this study predict that raising the palmar angle increases the load on the dorsal laminar junction. Therefore, hoof care interventions that raise the palmar angle in order to reduce the dorsal lamellae load may not achieve this outcome."

DRAFT

What are the benefits and disadvantages of barefoot v shod for horses recovering from laminitis?

We see a lot of horses fail to recover, or P3 penetrate the sole, when they are wearing heart bar shoes, either plastic or metal.
Some examples:

Horse&Hound Forum 2010
A 14 year old 16 hh mare had been suffering from laminitis for 10 weeks when she suddenly deteriorated and the pedal bone could be seen as a bulge in her sole.  X-rays were taken and confirmed severe rotation in both back feet with around 1 mm of sole covering the pedal bone.  Front feet were not affected.  Imprints were fitted and eased her discomfort, after which pain was only seen when she walked out of the stable.  On between 4 and 1 Bute a day, not tested for PPID (or insulin?). Mare described as always having sensitive/thin soles.  3 days after posting on a forum for help, the owner reported that the mare had been unable to put any weight on her worst foot, the bone had moved more and sunk and vet and farrier said there was no alternative but to PTS.

Lessons to learn:
X-rays should have been taken as soon as laminitis was suspected - ideally on the first day, and definitely within the first week - and the feet realigned and supported through the full solar surface.
Horses should not leave their area of confinement with wall to wall bedding until their feet are fully realigned, well supported through the full solar surface, and they are off of all pain relief.
Lifting a horse off of its soles will often reduce pain - but is also likely to leave the pedal bone unsupported, allowing actual solar penetration.
Shoes will load the walls - already disconnected from the bone - and not allow lowering of the heels but protection of sole depth in the front of the foot. 
The cause of the laminitis must be identified as soon as possible and removed/treated/managed.

Although very limited, research by Panagiotopoulou et al. at the RVC that looked at the effects of a steel shoe v no shoe on one TB gelding when walking over a platform, found that "in all cases, the shod horse showed higher concentration of stresses on the P3 than the unshod condition, with the caveat that none of these differences were statistically tested."
Panagiotopoulou O, Rankin JW, Gatesy SM, Hutchinson JR
A preliminary case study of the effect of shoe-wearing on the biomechanics of a horse’s foot
PeerJ. 2016;4:e2164. doi:10.7717/peerj.2164

Following laminitis horses can develop separation at the coronary band with drainage of fluid that may be bloody or clear serum.  In severe cases the whole hoof capsule can slough and become loose and even come off completely.

Hunt RJ
Equine Laminitis: Practical Clinical Considerations
AAEP PROCEEDINGS  Vol. 54  2008
Hunt suggests that "when steroid-induced laminitis does result in displacement of the coffin bone, it is generally severe and may result in sloughing of the hoof capsule in a matter of days to weeks."  Detachment of the hoof capsule may also be seen in horses with sepsis-related laminitis within days to weeks. 

"There are some extreme cases where sloughing of the entire hoof capsule is inevitable, facilitating the removal of the detached hoof capsule seems to bring relieve but nothing to nail or glue on to! Deep bedding, good bandaging and nursing can tie these patients over till they have grown a new hoof capsule back, a process which goes much quicker then one would expect."  See photos in link:
Hans Castelijns, farriery.eu

Hoof pulled off, loss of entire hoof capsule - horsesidevetguide.com


"Hoof sloughing can and does occur in more severe cases. Experience has shown that equines 14 hh upwards are more likely to succumb to this complication, due to carrying sufficient weight to more readily tear through the hoof. With such cases a second crisis period, post-acute stage, anywhere between two to four months into recovery is not uncommon. This is when the hoof capsule is sloughing but the new hoof has yet to regenerate and may be less than a quarter grown."
Treatment of Laminitis - Andrew Poynton, imprintshoes.co.uk

Cases where horses recovered from hoof sloughing

The Helpful Farrier: Dimpled Laminitis Treatment Stirs Facebook Furor and Charitable Shoeing - Fran Jurga, hoofcare.blogspot.com July 2013
Shows a photo of a laminitic foot (A month after receiving medical support...), with a line "where the mare had tried to slough her foot before she received treatment".
​

Case Report: Laminitis - Life Data Labs Inc. 
The hoof walls sloughed off of a 13 year old Quarter Horse with severe laminitis.  The horse made a full recovery and had returned to his previous level of work 18 months later.