VCOT May 2025 – Ortho Part 2: TTT Spacer Pins & The Working Length Myth

In this Simini Small Animal Surgery Podcast episode, we continue our orthopedic coverage from the May 2025 issue of Veterinary and Comparative Orthopaedics and Traumatology (VCOT) with two studies that challenge long-held assumptions about orthopedic fixation. 

One paper examines whether a novel spacer pin technique can provide the same stability as traditional tibial tuberosity transposition fixation while reducing implant-related complications. The second tackles one of the most debated concepts in fracture fixation: plate working length and whether leaving holes empty near a fracture gap truly provides biomechanical advantages.

In this episode:

✅ Sullivan et al. — An ex vivo biomechanical study comparing three fixation methods for tibial tuberosity transposition (TTT) in the treatment of medial patellar luxation. The authors evaluated traditional tension band wiring, dual-pin fixation, and a novel spacer pin technique. All constructs demonstrated comparable stiffness and failure strength, with each tolerating more than 1,000 Newtons of force before failure. Importantly, the spacer pin avoided implant placement directly through the patellar ligament insertion, potentially reducing soft tissue irritation and implant-associated complications while maintaining equivalent biomechanical stability. 

✅ Trefny et al. — A biomechanical investigation of working length in locking compression plate constructs using a distal radius fracture model. Contrary to conventional wisdom, constructs with short working lengths (screws placed adjacent to the fracture gap) were approximately 30% stiffer and generated lower plate strain than constructs with longer working lengths. While long working length constructs initially benefited from load sharing through transcortical contact, the authors highlight the biologic risks associated with repeated bone-end contact, including high interfragmentary strain, bone resorption, and eventual implant fatigue failure. 

Together, these studies demonstrate that orthopedic success often comes down to understanding where forces are actually being transmitted—and avoiding assumptions that may no longer hold true.

🎓 Journal Articles Discussed

• Sullivan et al. — Biomechanical Comparison of Spacer Pin Fixation to Two Established Methods of Tibial Tuberosity Transposition Stabilization in Dogs
• Trefny et al. — Effect of Plate Screw Configuration on Construct Stiffness and Plate Strain in a Synthetic Short Fragment Small Gap Fracture Model Stabilized with a 12-Hole 3.5-mm Locking Compression Plate

📚 From the May 2025 issue of VCOT

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Transcript

SPEAKER_00 0:04

Hi, I'm Carl Damiani, and this is the Simone Small Animal Surgery Podcast, your fast focused update on what matters most from the latest small animal surgical literature. In each episode, we break down key articles from the veterinary journals and translate them into surgical insight you can use, today, not someday. This episode continues our orthopedic coverage from issue 3, 2025, of veterinary and comparative orthopedics and traumatology, and we're diving into two studies that challenge some long-held assumptions about orthopedic fixation. First, we'll look at a biomechanical study by Sullivan et al. comparing a novel spacer pin technique for tibial tuberosity transposition against two established fixation methods used in the treatment of medial patellar luxation. The question is simple but clinically important. Can we achieve the same mechanical stability while potentially reducing some of the complications associated with traditional pins and tension band wiring? Then we turn to Trefney et al., who tackle one of the most debated topics in fracture fixation, plate working length. Using a locking compression plate fracture model, they examine how screw placement influences construct stiffness, plate strain, and load sharing across the fracture gap. Their findings offer valuable insight into why some constructs fail and whether leaving empty holes near a fracture really provides the biological and mechanical advantages we often assume. Two studies, one common theme, the details matter. Whether you're stabilizing a tibial tuberosity transposition or planning a bridge plating construct, small technical decisions can have major biomechanical consequences. Let's dive in.

SPEAKER_02 1:50

So uh picture this. You just closed what looked like a well, a flawless medial patellar luxation surgery.

SPEAKER_04 1:57

Right, the dream scenario.

SPEAKER_02 1:58

Aaron Powell Exactly. I mean everything lined up beautifully. But statistically, there is up to a uh 43% chance that dog is coming back with a complication.

SPEAKER_04 2:07

Which is just a massive number.

SPEAKER_02 2:09

Aaron Powell It really is. Yeah. So welcome to this deep dive. Today we are looking at a biomechanical tweak that could, you know, practically eliminate the hardware failures keeping you up at night.

SPEAKER_04 2:20

Aaron Powell Yeah. It really is one of the most frustrating clinical realities in veterinary orthopedics, right? I mean tibial tuberosity transposition or TTT, it's meant to fix a mechanical problem. But the traditional hardware creates a whole new set of headaches.

SPEAKER_02 2:33

Trevor Burrus, Jr. Right. Because we are talking about tension band wires and pins passing directly through the tuberosity.

SPEAKER_04 2:37

Trevor Burrus Exactly. Which leads to tuberosity fractures, pin migration, and just severe soft tissue irritation.

SPEAKER_02 2:43

Aaron Ross Powell Which brings us to our focus article today, uh Sullivan et al. 2025. This ex vivo cadavric study really feels like a quest to see if we can finally ditch that metal heavy standard.

SPEAKER_04 2:54

Aaron Powell That's a good way to put it. They essentially wanted to know if a newer spacer pin technique could actually hold up under acute tension uh compared to the established methods.

SPEAKER_02 3:04

Aaron Powell Okay, so how do they actually test this? Like what was the setup?

SPEAKER_04 3:07

Well, they tested three groups to failure. You had the traditional complete osteotomy with a tension band wire, then a partial osteotomy with two pins, and finally Let me guess.

SPEAKER_02 3:18

The spacer pin.

SPEAKER_04 3:19

Yeah, exactly. A partial osteotomy using just a single sponsor pin placed medial to the tuberosity.

SPEAKER_02 3:25

Okay, let's unpack that spacer pin concept for a second. Yeah. Because I mean, in instead of nailing the tuberosity segment down directly through the bone, it's uh it's almost like sliding a structural wedge or a doorstop behind it to hold the transposition.

SPEAKER_03 3:38

Yeah, the doorstop analogy is perfect. It's a doorstep that completely changes the angle of tension.

SPEAKER_02 3:43

But I'm skeptical. I mean, the quadriceps muscle is incredibly powerful. Are you telling me a single wedge medial to the tuberosity can withstand the same explosive force as a wire literally wrapping it down to the bone?

SPEAKER_04 3:55

Well, instead of bearing the brunt of the pole directly, that wedge leverages the intact bone below it to diffuse the force. And uh crucially, you completely avoid placing pins directly through the insertion point of the patellar ligament.

SPEAKER_02 4:11

Oh wow, right. So you are sidestepping a major source of tissue trauma right from the start.

SPEAKER_04 4:16

Exactly.

SPEAKER_02 4:17

But does the data actually back that up? Like, does this wedge hold when you really pull on it?

SPEAKER_04 4:22

It absolutely does. Sullivan et al. 2025 found no significant difference in ultimate failure force or stiffness among any of the three groups.

SPEAKER_02 4:30

Wait, none at all. So how does the bone handle that tension without the wire wrapping it down?

SPEAKER_04 4:35

It comes down to the osteotomy itself. When you leave a robust distal tibule crest intact during your cut, the bone and the periosteum are inherently fighting that upward pull of the quadriceps.

SPEAKER_02 4:46

Ah, so the animal's own anatomy acts as a like a physiologic tension band.

SPEAKER_04 4:51

Exactly. The bone itself is doing the job of the metal wire.

SPEAKER_02 4:54

Aaron Powell Okay, so the anatomy works. But how strong is the construct overall?

SPEAKER_04 4:58

Aaron Powell Impressively, all three constructs withstood over 1,000 newtons of force.

SPEAKER_02 5:02

Aaron Powell Wait, 1,000 newtons? That is well above normal walking loads.

SPEAKER_04 5:06

It really is. And the most common mode of failure across the board wasn't the bone breaking or the implant snapping. It was actually the patellar ligament tearing.

SPEAKER_02 5:15

Aaron Powell That is wild. The soft tissue gave out before the spacer pin did. But you know, to play devil's advocate here, acute failure in a cadaver lab isn't exactly the same as cyclic loading.

SPEAKER_04 5:26

Right, that's true.

SPEAKER_02 5:27

Like picture a dog jumping on and off a sofa 30 times a week during recovery.

SPEAKER_04 5:32

That's a highly valid point. The lab proves the baseline mechanical integrity is solid, though. It proves you don't actually need the wire for acute strength.

SPEAKER_02 5:40

So it's part of a broader shift in modern surgery, right? Moving away from mechanical overkill and towards biological preservation.

SPEAKER_04 5:46

Yes, exactly. You want to leave the site as undisturbed as possible.

SPEAKER_02 5:49

Aaron Powell Right. But preserving biology isn't just about using less metal. I mean, speaking of eliminating hidden risks, if you leave the surgical site a microscopic mess, your perfectly placed spacer pin won't really matter. Infection is just as frustrating as an implant failure.

SPEAKER_04 6:05

Oh, 100%. Which is why reinforcing your protocol with something like uh Seminy Protect Livage makes a lot of sense here. Trevor Burrus, Jr.

SPEAKER_02 6:12

Right, because standard saline actually leaves a lot behind.

SPEAKER_04 6:15

It does. In independent head-to-head studies, standard saline left 42% of bacteria behind in the surgical site. Semony protect lavage left zero. Trevor Burrus, Jr.

SPEAKER_02 6:24

Leaving almost half the bacteria behind with just a saline rinse is a massive blind spot.

SPEAKER_04 6:29

It really is. And since simony is a non-antibiotic lavage, it aligns perfectly with antimicrobial stewardship. You just take 60 seconds prior to suturing to use an evidence-based lavage, and it removes what saline misses.

SPEAKER_02 6:42

Aaron Ross Powell It's all about working smarter and protecting that carefully preserved biology, giving you peace of mind in the OR. So uh what is the ultimate take-home message from Sullivan et al. twenty twenty five?

SPEAKER_04 6:53

The clinical punchline is basically this the spacer pin technique is a biomechanically viable, strong alternative to the traditional tension band wire for TTT stabilization.

SPEAKER_02 7:04

As long as you preserve that distal crest.

SPEAKER_04 7:06

Exactly. Provided you preserve that distal crest to act as your tension band, it can seriously spare your patients from implant-related soft tissue irritation.

SPEAKER_02 7:14

Trust the construct. I love it. But I want to leave you with one final thought to mull over, tying back to what we touched on earlier. If all these surgical constructs can handle over a thousand newtons of acute force in a controlled lab without the implants failing, uh, how much of our clinical complication rate in the real world is actually just a failure of owner compliance and cyclic loading during post-op recovery?

SPEAKER_01 7:41

Let's explore another relevant study.

SPEAKER_04 7:49

Um picture your or are tomorrow. You have a dog on the table with a broken leg, like specifically a distal radius fracture.

SPEAKER_02 7:57

Right. A very common scenario.

SPEAKER_04 7:58

Exactly. It's a short fragment break, and you just, you know, you can't get the pieces to line up perfectly. So you have your locking compression plate ready to go. Yeah. But before you place your screws, you have a choice. Do you leave the holes near that fracture gap empty to increase your working length, or do you just fill them all?

SPEAKER_02 8:14

And that is exactly the dilemma we are looking at today.

SPEAKER_04 8:17

Right. Today's mission is a deep dive into a biomechanical study by Trefney at AL2025 to pull out a clear, actionable punchline you can actually use the next time you face this.

SPEAKER_02 8:29

Yeah, because this specific setup, uh, a short fragment fracture with a tiny 1.75 millimeter gap is something you face constantly.

SPEAKER_01 8:37

Definitely.

SPEAKER_02 8:38

The researchers use synthetic bone models to test 12-hole locking compression plates under four-point bending.

SPEAKER_04 8:44

Okay.

SPEAKER_02 8:45

And they've basically compared short working lengths against long ones.

SPEAKER_04 8:48

Aaron Powell Well, intuitively, I always picture a bone plate like a bridge spanning a river, right? If you leave the plate holes near the gap empty, you're building a longer bridge. You'd think a longer bridge distributes stress better, keeping the implant from taking all that localized force.

SPEAKER_02 9:04

That makes perfect sense in theory.

SPEAKER_04 9:05

Aaron Powell Right. So if a longer bridge is supposed to be better, did the lab data actually show the long working length failing when the bone was compressed?

SPEAKER_02 9:14

Well, no, it actually flipped that intuition completely upside down.

SPEAKER_04 9:17

Wait, really?

SPEAKER_02 9:18

Yeah. In compression bending, the short working length, you know, filling those holes near the gap was the clear winner. Oh wow. The short construct was roughly 30% stiffer than the long one.

SPEAKER_04 9:29

30%? That's huge.

SPEAKER_02 9:31

Yes, and it took significantly less strain on the plate itself.

SPEAKER_04 9:34

Okay, that feels completely backward. I mean, if the shorter bridge is stiffer and takes less strain, why is there even a debate? Well there has to be a reason some surgeons still advocate for leaving those holes empty.

SPEAKER_02 9:46

Uh the debate really stems from what happens when the bone is under tension rather than compression.

SPEAKER_04 9:53

Ah, okay. That makes sense.

SPEAKER_02 9:55

Let's go back to your bridge analogy. Imagine that longer bridge sagging under the weight of heavy traffic until the middle of the bridge actually hits the rocks in the river below.

SPEAKER_04 10:05

Right, it bottoms out.

SPEAKER_02 10:06

Exactly. In the bone, we call that transcortical contact. The bending forces cause the fracture gap to close until the bone ends literally touch.

SPEAKER_04 10:17

So when the bone ends finally touch, the bone itself starts carrying the weight instead of just the plate.

SPEAKER_02 10:23

Precisely.

SPEAKER_04 10:24

And the lab data showed that right around 150 newtons of force, the long working length suddenly gets highly stable because of that load sharing, right?

SPEAKER_02 10:32

It does. The moment those bone ends touch in the lab, construct stiffness spikes and plate strain just drops dramatically.

SPEAKER_04 10:40

Aaron Powell, but hold on. If the lab shows the long working length stabilizing and taking strain off the plate once the bones touch, wouldn't we want that? I mean, I think encouraging load sharing would protect the implant.

SPEAKER_02 10:51

Well, in a single load lab test, it looks great, but you have to translate this to a living walking dog.

SPEAKER_04 10:56

Right.

SPEAKER_02 10:57

Every single time that dog takes a step, those bone ends are bumping together and then pulling apart. Ouch. Yeah, that seplik transcortical contact creates what we call 100% interfragmentary strain right at that contact point.

SPEAKER_04 11:12

And here is where the biology ruins the mechanical theory, right? Trevor Burrus, Jr.

SPEAKER_02 11:16

Completely.

SPEAKER_04 11:16

Because a 100% strain environment is totally toxic for bone healing. With every step, that constant micro motion is literally crushing and tearing the fragile new bone cells trying to bridge the gap.

SPEAKER_02 11:29

Exactly. The body can't build bone in that chaos.

SPEAKER_04 11:32

Aaron Powell So it does the opposite. It just resorbs the bone at the fracture ends.

SPEAKER_02 11:36

Right, which means your patient resorbs the bone, the gap widens, and that helpful transcortical contact disappears completely.

SPEAKER_01 11:42

Oh, I see.

SPEAKER_02 11:43

Now your plate is taking all the strain again. But because you left those holes empty, you have a long working length that flexes more with every step.

SPEAKER_04 11:52

Aaron Powell Which is a fast track to implant fatigue failure.

SPEAKER_02 11:54

You got it.

SPEAKER_04 11:55

So the takeaway for the surgeon scrubbing in tomorrow is crystal clear. If you're dealing with a small gap, imperfectly reduced fracture, do not intentionally leave holes empty to increase your working length.

SPEAKER_02 12:06

Absolutely. Fill those holes. Right. The short working length gives you higher stiffness and lower strain, and it keeps you out of that biological trap of high interfragmentary strain and bone resorption.

SPEAKER_04 12:16

Perfect. The link to Trepne all 2025 is in the show notes for you to review. But before we wrap up this deep dive, think about this. Yeah. If leaving empty holes is so detrimental for small gap fractures because it causes that toxic bone-on-bone contact, how does this data force us to rethink screw placement strategies in highly commutative fractures where transcortical contact is impossible to begin with? Something to mull over.

SPEAKER_01 12:45

That's it for this episode of the Semini Small Animal Surgery Podcast. This show is brought to you by Semini Protect Livage, our interoperative lavage developed to target resistant bacteria and biofilms where traditional solutions of saline and post op antibiotics fall short. If you're interested in learning more or trying out your own procedures, you'll find information and links in the show notes. For listening, and we'll see you in the next episode.