This short but strong band of collagen runs inside the stifle joint – the equivalent to the human knee – and keeps it together. This is important because a stifle lacks intrinsic stability.

While a hip is a ball and socket, a bit like a tow bracket for a caravan or trailer, a stifle comprises the femur (thigh bone) just resting on the tibia (shin bone). The teardrop sliver of the patella, the tiny, mint imperial-like fabellae and the diminutive fibula add to the bony arrangement, but not really to joint stability. Without the integrity of the ligaments inside the joint everything goes wrong.

At vet school, we were taught that when a fast-moving spaniel puts his hind foot down a rabbit hole and comes to a sudden stop, forward momentum was the main cause of the injury. The lower part of the limb stays put and the body continues at 20mph. The poor cruciate ligament doesn’t stand a chance.

We also see lameness caused by weakness in this area. A weak ligament can fray or tear even without a sudden with force on the joint.

An experienced vet looking at a Rottweiler, sad and toe-touching on his back leg, would immediately look at the stifle.

Even before we switch on the X-ray machine, a thorough examination can be enough to make a diagnosis. A fully torn cranial cruciate ligament leaves a slack joint, where the two important bones can be pushed against each other; it’s called a cranial drawer test. With experience it’s easy to appreciate, but it’s a technique that is tricky to learn.

The diagnosis might be easy, but the treatment causes bigger problems because there are a multitude of options available.

Small dogs with mild injuries can do well with conservative treatment – rest, anti-inflammatories and time. But a complete tear in an energetic large dog has little chance of resolution without some sort of surgery; at least, not without the development of a degree of osteoarthritis, as the body tries its best to develop some natural stability. It can do so up to a point, but not without long-term pain and disability.

So, for many cruciate ligament injuries, the discussion between the vet and anxious owner turns to the surgical options. There are several alternatives and no one solution stands head and shoulders above the rest.

The simplest, called a lateral suture, involves adding an artificial nylon ligament, which hugs the stifle and affords an almost immediate stability. It is quite straightforward to perform and relatively inexpensive as the prosthetic ligament is basically a modified version of thick fishing line, its placement and fastening facilitated by some fancy tools.

At the other end of the spectrum is the TPLO, or a tibial plateau levelling osteotomy, which is as complicated and expensive as it sounds. For various reasons, it is the technique reserved for specialist orthopaedic surgeons, who can screw together spines and replace hips.

To make decision-making harder still, there is another option, sitting between basic and top-of-the-range. It’s called a TTA. It is fiddly, and requires titanium things but, in my view, for hyperactive springer spaniels, it is most definitely the way forward. For Holly, the energetic springer who I operated on recently, it turned out to be the perfect solution too.

The US Vet continues on Tuesdays at 8pm on Channel 5.