The Physiology of Orthopaedic Surgery

An Introduction to the Physiology of Surgery

Undergoing orthopaedic surgery is a significant milestone on your journey to better health. While it’s natural to feel stressed going into it, knowledge is a powerful tool for your recovery. It can be tempting to simply "tune out" until the procedure is over, but understanding how your body responds to surgery is one of the best ways to feel prepared and confident.

Why Understanding Surgery Can Help You Feel More Prepared

Grasping the science behind a surgery transforms the experience from something happening to you into a process you are actively navigating. Think of it like a map for a hike; it doesn't make the mountain any flatter, but it helps you know exactly where the steep parts are and how to pack your bag accordingly.

With that in mind, let's get into the science and build our "map" for understanding surgery recovery.

The Surgical Stress Response

For every surgery, no matter how carefully planned or successful, the body sees the incision and tissue repair as a major event that needs an immediate reaction. To protect you, your system starts a process called the surgical stress response. This response is a normal and temporary part of how your body works to fix itself.
It’s helpful to see this response as a natural survival habit. Your body is trying to get back to "homeostasis"—or internal balance—as quickly as possible after an injury. During this time, your body makes two main internal shifts:

The Neuroendocrine Response: Your body releases hormones like cortisol and catecholamines to help manage swelling and change blood flow. This helps deliver more nutrients and oxygen to the wound. It also shifts your metabolism into "repair mode" to start creating more energy. To do this, your body breaks down its own stores of protein, fat, and carbohydrates to get the materials it needs to support your immune system and help the wound heal.
The Inflammatory-Immune Response: Your immune system sends out special cells and proteins (called cytokines) to the surgical site. Their job is to prevent infection and start the process of knitting the tissues back together.

These responses make up your surgical stress response, which is overall very important to your healing process. However, it is also very "expensive" for your body because it uses up a lot of materials and energy from your internal systems.

Surgery's Impact on Your Body

As your body works overtime to heal, its need for nutrients goes up significantly:

Oxygen Demand: Major surgery can increase your body’s need for oxygen by as much as 50% to support the work of your liver and muscles (1). This is why exercising and improving your heart and lung health can be so helpful—it helps them deliver that extra oxygen more efficiently.
Energy and Nutrient Reserves: To fuel the repair process, your body breaks down its own stored energy, including fats and proteins. The proteins can be taken directly from your muscle. If a patient has low energy reserves—which sometimes happens with malnutrition or muscle loss (sarcopenia)—the body has fewer "building blocks" to use. This is why protein recommendations for surgical patients can be double what they usually are.
Protein As a Fuel Source: Your liver uses amino acids (protein) from your muscle to create energy, ensuring your brain and healing tissues have constant fuel. This is why fasting for a long time before surgery can sometimes make the stress response worse; your body feels the pinch of having fewer resources sooner.

Preparing with Confidence

Now that you know what’s happening under the surface, you can see that recovery doesn't actually start in the physical therapy office; it begins in the weeks leading up to your procedure. By supporting your body’s reserves now, you help it manage the stress response more effectively later.

Actionable Tips to Take Charge:

Build Your "Bank Account" of Nutrients: Since surgery increases the demand for proteins to repair tissue, focusing on high-quality protein sources (like lean meats, beans, yogurt, or high protein supplements) in the weeks before surgery can help ensure your body has the amino acids it needs.
Focus on Cardiovascular Health: If cleared by your doctor, light aerobic activity (like walking) can improve your heart and lung capacity, making it easier for your body to meet the increased oxygen demand (2).
Mind the Micros: Nutrients like vitamin C and zinc are essential for wound healing. Ensuring that you’re eating a colorful variety of fruits and vegetables helps provide the tools your immune system needs to do its job.
Nutrition Should Continue After Surgery Too: While we covered the importance of nutrition to prepare your body for your procedure, it is integral to continue consuming high protein after your procedure, as recovery can take 6-12 months. Your body will still need more resources as it repairs itself.

While your internal systems handle healing your body, you can provide the fuel and the environment they need to succeed. Now that you understand the science, you’re ready to actively build the foundation for your own recovery.

Scientific References:

Hirsch KR, Wolfe RR, Ferrando AA. Pre- and Post-Surgical Nutrition for Preservation of Muscle Mass, Strength, and Functionality Following Orthopedic Surgery. Nutrients. 2021;13(5):1675.
Gillis C, Ljungqvist O, Carli F. Prehabilitation, enhanced recovery after surgery, or both? A narrative review. Br. J. Anaesth. 2022;128(3):434-448.

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