Microscope Extenders for Dentists: A Practical Ergonomics Upgrade That Protects Your Neck, Back, and Workflow

May 20, 2026

Why “better posture” often starts with the microscope setup—not the clinician

Dental professionals spend hours in fixed positions, making small, repetitive adjustments under magnification. Over time, those micro-compromises add up—especially when you’re craning to meet the oculars, losing neutral head posture, or constantly “hunting” for the right viewing position. A properly selected microscope extender can be one of the most effective, low-disruption ways to regain a comfortable working distance, improve positioning flexibility, and reduce fatigue without replacing your entire microscope system.

DEC Medical perspective

DEC Medical has supported the New York medical and dental community for over 30 years with surgical microscope systems, accessories, and—most importantly—real-world integration help. Extenders and adapters are often the difference between a microscope that’s “technically compatible” and one that’s genuinely comfortable and efficient day after day.

Relevant pages: Products · Microscope Adapters · CJ Optik · About DEC Medical

What is a microscope extender (and what problem does it solve)?

A microscope extender is an accessory component that adds height/length at a specific point in the optical or mechanical chain (depending on system design). In dental operatory terms, it’s often used to help align the microscope’s viewing geometry with your natural posture—so you can keep a neutral head and neck position while maintaining the working distance you need for the procedure.

When the microscope’s geometry doesn’t match the clinician and operatory layout, the common “workarounds” are predictable: leaning forward, elevating shoulders, tilting the head back/forward, or seating adjustments that feel fine for five minutes and punishing after five hours. Ergonomics research consistently points to awkward or sustained postures as a major risk factor for work-related musculoskeletal disorders (MSDs). An extender is an engineering control-style fix: it changes the equipment configuration so the body doesn’t have to compensate.

Where extenders help most in dental microscopy

1) Neutral head/neck posture at the oculars

If you’re raising your chin to reach the oculars (or dropping your head and rounding your shoulders), you’re spending the procedure in compensation mode. Extenders can help bring the oculars to you—rather than forcing you to meet them.

2) Stable working distance across procedures

Endodontics, restorative dentistry, and surgical workflows often require long, steady periods under the scope. When working distance is inconsistent, your posture becomes dynamic in the worst way: constant micro-adjustments that create fatigue.

3) Multi-provider operatories

If more than one clinician uses the same operatory, extenders (paired with the right adapters) can make it easier to “reset” the scope quickly—reducing wasted time and improving consistency from provider to provider.

How to tell if you need an extender (quick self-check)

If any of these feel familiar, an extender is worth evaluating:

Your posture changes when you “go to the scope”

You can sit upright for setup and assistant communication, but the moment you place your eyes at the oculars, your head/neck drifts out of neutral.

You lose comfort at higher magnification

Higher magnification narrows tolerance. If you feel “locked in” with tension, the geometry and reach may not be matched to your working distance.

You’re adjusting chair/patient position to accommodate the microscope

Patient and clinician positioning should support access and airway—then the microscope should be configured around that reality (not the other way around).

Step-by-step: choosing microscope extenders for dentists (without guesswork)

Step 1: Define your “neutral posture” target

Before measuring hardware, confirm what you’re aiming for: relaxed shoulders, supported spine, and a head position that stays neutral when your eyes are in the oculars. If you need to flex or extend the neck to see clearly, you’re starting from a compromise.

Step 2: Map your current constraints (room + mounting + patient positioning)

Extenders don’t live in isolation. Ceiling mount vs wall mount vs floor stand, operatory ceiling height, chair range of motion, and where assistants need to work all influence what “better ergonomics” can look like in the real room.

Step 3: Confirm compatibility points (this is where adapters matter)

Many practices have a microscope from one manufacturer, mounting or accessory components from another, plus camera ports, beam splitters, or custom lighting. That’s why microscope adapters are frequently paired with extenders—to ensure mechanical fit and maintain intended alignment. If you’re integrating across systems, start with DEC Medical’s adapter options as a reference point for what’s possible.

Step 4: Decide whether you’re optimizing ergonomics, workflow—or both

Some extenders are chosen primarily to reduce fatigue (bringing oculars into a more comfortable zone). Others help standardize reach and positioning for repeatable setups, especially if you’re documenting cases or sharing operatories. Clarifying the “why” keeps the configuration clean and avoids stacking accessories that don’t add value.

Common extender vs. no-extender outcomes (quick comparison)

What you notice	Often seen without an extender	Often improved with the right extender
Head/neck comfort at oculars	Chin up/down, neck tension, shoulder elevation	More neutral posture; less “reaching” to see
Time spent re-positioning	Frequent micro-adjustments; “hunting” for oculars	Faster setup; steadier working zone
Multi-provider consistency	Each provider compensates differently	Easier “reset” between clinicians
Integration with other accessories	Fitment limitations; awkward stacking	Cleaner geometry when paired with proper adapters

Note: exact results depend on microscope model, mounting type, working distance, and how the system is configured (objective, tube, beam splitter/camera components, and operator posture habits).

Did you know? Quick facts that matter for dental ergonomics

MSDs include the neck and back. Work-related musculoskeletal disorders can affect muscles, tendons, ligaments, nerves, joints, and other structures—often aggravated by sustained or awkward postures.

Small angles matter. Even modest, sustained neck flexion can increase muscular load and fatigue during microscope work—especially when sessions are long and repetitive.

Ergonomics is an equipment issue and a habits issue. An extender can correct geometry, but training your workflow (patient positioning, assistant coordination, and scope placement) helps the improvement stick.

Where DEC Medical fits: matching the right extender to the real operatory

Extenders are most successful when they’re selected with the full system in mind: your microscope brand/model, how it’s mounted, the procedures you do most often, and how you (and your assistants) naturally move around the patient. DEC Medical’s focus on adapters and extenders is practical: practices don’t always need a full replacement microscope—they need a better interface between the microscope they already trust and the way they actually work.

If you’re exploring a full system upgrade as well, DEC Medical also distributes premium microscope systems, including CJ Optik microscopes, and supports accessory integration through their products catalog.

Local angle: New York expectations—fast schedules, tight rooms, multiple providers

Even though DEC Medical serves nationwide needs, New York operatories often share a few realities: limited space, busy schedules, and teams rotating between rooms. In that environment, ergonomics upgrades need to be repeatable. A microscope extender can help standardize a “known good” viewing position so you spend less time re-configuring between patients—and more time working comfortably and consistently.

If you’ve ever found that one operatory “feels great” and another feels like a fight, that’s usually not a mystery. It’s geometry: mounting location, chair range, and how the microscope reaches the field. Extenders and adapters are designed to close that gap.

Talk to DEC Medical about microscope extenders for dentists

If you want help selecting an extender that matches your microscope and operatory layout, DEC Medical can guide the configuration so you get an ergonomic improvement you can actually feel—without creating new fitment or workflow issues.

Request extender & adapter guidance

Prefer to browse first? Visit Products or learn more about DEC Medical.

FAQ: microscope extenders for dentists

Do microscope extenders change image quality?

A properly designed extender used as intended should preserve alignment and usability. The key is compatibility and correct installation—especially when multiple accessories are involved (beam splitters, cameras, inclinable tubes, or custom mounts). That’s where pairing extenders with the correct adapters matters.

Is an extender only for tall clinicians?

Not at all. Height is only one variable. Extenders can help anyone whose microscope reach, ocular position, mounting location, or chair/patient positioning forces awkward posture—regardless of clinician height.

Can I use an extender with my existing microscope brand?

Often yes, but it depends on the microscope’s configuration and the connection points. If you’re integrating across manufacturers (or adding components like a camera adapter), you’ll likely need a matching adapter solution to ensure fit and stability.

What’s the difference between a microscope extender and an adapter?

An extender typically changes reach/height/spacing to improve positioning and ergonomics. An adapter is primarily about compatibility—connecting components between systems or standards. Many ergonomic improvements use both: adapters for fit, extenders for geometry.

What information should I have ready before requesting help?

Your microscope make/model, mounting type (ceiling/wall/floor), any existing accessories (camera port, beam splitter, inclinable tube), and a description of what feels “off” (neck flexion, shoulder elevation, limited reach). Photos of the operatory setup can also speed up recommendations.

Glossary (quick definitions)

Microscope extender

An accessory component that adds spacing/height at a connection point to improve reach and ergonomic positioning.

Microscope adapter

A compatibility component that connects parts between different manufacturers, standards, or mounting/accessory systems.

Working distance

The distance from the microscope optics to the treatment field where focus and posture can be maintained comfortably.

Neutral posture

A body position with minimal strain: head stacked over shoulders, relaxed shoulders, and a supported spine—reducing sustained muscular load.

MSD (Musculoskeletal disorder)

A condition affecting muscles, tendons, ligaments, nerves, joints, or supporting structures that can be caused or aggravated by work conditions and posture.

50 mm Extender for Global Microscopes: When It’s the Right Ergonomic Fix (and When It Isn’t)

May 6, 2026

A small spacer can change posture, access, and daily comfort more than most upgrades

Dental and medical clinicians often assume discomfort at the microscope means “I need a different scope.” In reality, many issues are geometric: the binoculars sit just a bit too close, an accessory stack shortens usable reach, or an assistant/camera configuration crowds the working zone. A 50 mm extender for Global microscopes is one of the simplest ways to restore clearance and regain a neutral working posture—without forcing a full system replacement. DEC Medical supports practices across the United States with microscope extenders and adapters designed to improve ergonomics and compatibility while keeping your workflow consistent.

What a “50 mm extender” actually does

A 50 mm extender is a precision spacer that adds 50 millimeters of length between microscope components (commonly between the binocular tube and the microscope body, or within a configured accessory “stack,” depending on the system). That added length can:

Improve clearance for hands, instruments, and retraction—especially when a camera/beam splitter/assistant scope is involved.
Support neutral posture by reducing the “lean-in” habit that creeps in when optics feel just out of reach.
Stabilize your working setup so different clinicians can maintain a repeatable position across operatories.

Ergonomics guidance in dentistry consistently emphasizes reducing sustained neck and upper-back strain through neutral positioning and properly set working distance—microscope geometry is a major lever for that.

The most common problems a 50 mm extender solves in a Global setup

If you’re running a Global microscope, an extender is often considered when the microscope is optically excellent, but the physical relationship between clinician, patient, and optics feels “off”. Here are typical pain points where 50 mm makes a noticeable difference:

1) You keep creeping forward to “meet” the binoculars

If you regularly find yourself drifting out of the chair back support, flexing your neck, or rounding shoulders to stay in the oculars, the optics may be positioned too close/too low relative to your seated posture. Adding length can help bring the viewing position back into a more sustainable alignment.

2) Your accessory stack reduced clearance

Adding documentation (camera), co-observation (assistant scope), or other modules can subtly change the geometry. An extender can restore space so your hands and instruments aren’t competing with the microscope head for the same real estate.

3) You’re trying to standardize rooms or providers

Group practices and multi-provider clinics often want a repeatable setup. A properly selected extender helps reduce “custom posture fixes” (extra cushions, awkward chair height changes, constant arm repositioning) that vary from room to room.

When a 50 mm extender is not the right first move

Extenders are powerful, but they’re not magic. Consider these situations before committing:

The microscope isn’t positioned correctly yet. Many “I need hardware” complaints are solved with arm positioning, chair height, patient positioning, and monitor placement.
You really need a working distance change, not a spacer. If your core issue is objective working distance (how far the scope focuses from the tooth), you may need an objective/variofocus solution rather than a length extender.
You’re fighting head angle, not reach. If your binocular angle forces neck flexion, a binocular extender or angled tube solution may be more effective than adding 50 mm elsewhere.

The best outcomes come from matching the accessory to the real constraint: reach, angle, clearance, or compatibility.

Step-by-step: How to decide if you need a 50 mm extender (clinic-friendly checklist)

Step 1: Confirm your “neutral baseline.”
Sit fully back, feet stable, elbows close to your body. If you can’t stay there while viewing, note what forces you out (neck bend, shoulder elevation, reaching).

Step 2: Identify what changed.
Did discomfort start after adding a camera, beam splitter, assistant scope, or new operator/stool? Geometry shifts often follow accessory changes.

Step 3: Evaluate clearance at the patient.
If you’re bumping the microscope head with your hands, mirror, ultrasonic, or retractors, you’re dealing with a spacing problem—an extender is often a strong candidate.

Step 4: Confirm the connection points.
“50 mm extender for Global” can mean different placement points depending on your configuration. The correct extender must match your exact interface and accessory stack.

Step 5: Verify asepsis workflow compatibility.
Any accessory should support your wipe-down routine and barrier strategy without creating hard-to-clean geometry. Follow your facility protocols and manufacturer instructions for reprocessing/cleaning of components and accessories.

Step 6: Standardize settings after install.
Once spacing is corrected, lock in chair height ranges, patient chair positions, and microscope arm “home” positions for consistency across providers.

Did you know? Quick microscope ergonomics facts

Neutral posture is a systems problem. The microscope can support your posture, but only if working distance and component placement don’t force head/neck compensation.

Accessory stacks change real-world geometry. Cameras, beam splitters, and assistant scopes can alter clearance and where you “end up” sitting—even if the optics are unchanged.

Consistency reduces fatigue. When your operatory setup is repeatable, you spend less time micro-adjusting your body and more time operating with stable hand positioning.

Quick comparison table: Extender vs adapter vs objective change

Upgrade type	Primary purpose	Best for	Watch-outs
50 mm extender	Adds length/space between components	Clearance issues, reach/stack geometry, posture “creep”	Must match interfaces; placement matters; confirm full configuration
Microscope adapter	Connects components across brands/standards	Compatibility (mixing accessories, modernizing parts)	Fitment details are critical (model, interface, accessory stack)
Objective / variable working distance	Changes focusing distance range to the field	When the tooth feels too close/far despite good clearance	May require different workflow habits; confirm compatibility

Note: Final recommendations depend on your exact microscope model, arm type, and accessory stack (documentation, assistant scope, beamsplitter, etc.).

How DEC Medical helps you spec the right extender (without guesswork)

Ordering microscope accessories shouldn’t feel like trial-and-error. The fastest path to a correct match is to gather a few details before you reach out:

Microscope brand/model (Global configuration details matter).
Current stack: binocular tube type, any beam splitter, camera, assistant scope, and objective.
Your constraint: clearance (hands/instruments), posture (neck/shoulders), reach (positioning), or compatibility (mixing components).
Operatory realities: chair type, typical procedures, left/right-handed use, and whether multiple clinicians share the room.

If you’re also evaluating adapters or a broader ergonomics refresh, explore DEC Medical’s product ecosystem for microscopes and accessories, or learn more about the company’s approach on the About DEC Medical page.

United States clinic angle: scaling ergonomics across multiple operatories

Across the United States, multi-location practices and DSOs often face the same challenge: microscopes are added gradually, rooms evolve, and accessory configurations become inconsistent. Extenders and adapters can be a practical way to standardize the “feel” of the microscope from room to room—so clinicians don’t spend the first 15 minutes of each procedure re-learning posture and positioning.

A useful internal standard is to document (1) typical chair height range, (2) patient chair tilt for key procedures, and (3) microscope arm “park” and “working” positions. Once your geometry is corrected, these standards become easier to maintain.

CTA: Get the right 50 mm extender for your Global configuration

If you’re considering a 50 mm extender for Global, a quick fitment check can prevent mismatches and help you solve the real ergonomic constraint (clearance vs reach vs angle vs compatibility). Share your microscope model and current accessory stack, and DEC Medical will help you narrow the correct solution.

Contact DEC Medical Explore Microscope Adapters

FAQ: 50 mm extender for Global microscopes

Will a 50 mm extender change image quality?

A properly specified extender is primarily a mechanical/geometry change. Image quality concerns typically come from mismatched optical components or incorrect interfaces. The key is correct fitment to your model and accessory stack.

Is a 50 mm extender the same thing as a binocular extender?

Not always. “Extender” can refer to different spacer locations. A binocular extender specifically adjusts the binocular viewing geometry; other extenders may sit elsewhere in the stack to restore clearance and reach.

How do I know if my issue is working distance vs clearance?

If you can focus well but keep bumping the scope head with your hands/instruments, that’s typically clearance. If you feel like the tooth is consistently “too close” or “too far” for comfortable posture even when you have space, that may point toward objective working distance.

Can a 50 mm extender help with neck pain?

It can—when neck strain is coming from “leaning in” to reach the oculars or from cramped accessory geometry. If neck pain is driven by viewing angle, you may need a different binocular configuration or posture/positioning changes.

What information should I send DEC Medical to confirm compatibility?

Send your microscope brand/model, photos of the current head/accessory stack (camera/assistant scope/beam splitter), and describe the exact problem (clearance, reach, posture, or compatibility). That typically allows fast, accurate matching.

Glossary (helpful terms when discussing extenders and adapters)

Extender (Spacer): A component that adds length between microscope modules to change clearance and geometry.

Adapter: An interface that allows components from different systems/standards to connect correctly.

Working distance: The distance from the objective lens to the treatment field where the image is in focus.

Accessory stack: The combined set of modules mounted together (e.g., binocular tube + beam splitter + camera + assistant scope).

Neutral posture: A sustainable working posture where the head/neck is not forced into sustained flexion and shoulders are not elevated or rounded to maintain view.

Dental Microscopes & Ergonomics: How to Build a Neck-Friendly Operatory Without Replacing Your Entire Setup

May 5, 2026

A practical guide to posture, positioning, and smart upgrades for clearer vision and less fatigue

Dentistry is precision work performed in tight spaces—and too often, it’s performed in a posture your body “pays for” later. Dental microscopes can improve visualization and support a more neutral working posture when set up correctly, but the real difference comes from the total system: microscope + mounting + adapter/extender choices + room layout + daily habits. This guide breaks down how to evaluate your operatory ergonomics and where microscope adapters and extenders can make a high-impact improvement without forcing a full equipment overhaul.

Why this matters: Work-related musculoskeletal disorders (MSDs) are widely reported among dental healthcare providers, with research summaries showing high overall prevalence—often cited around “seven out of ten” providers experiencing issues. (pmc.ncbi.nlm.nih.gov)

What a dental microscope can (and can’t) fix

Magnification is often discussed as “better vision,” but the daily win for many clinicians is posture support. Unlike head-worn magnification, a dental operating microscope (DOM) is adjustable and not carried on your head, and it can help you maintain a more upright position when properly configured. (agd.org)

The important nuance: ergonomics is a system, not a single device

A microscope can enable neutral posture, but only if the working distances, chair height, patient positioning, and microscope reach are dialed in. If the scope can’t comfortably reach the correct field without you “chasing it,” you’ll still end up with forward head tilt, elevated shoulders, or twisted trunk—just with better lighting.

Microscope adapters & extenders: the overlooked ergonomic upgrade

If you already own a microscope (or you’re planning to add one), adapters and extenders can be the difference between “I have a microscope” and “my microscope fits my body and room.” In many operatories, constraints like ceiling height, light booms, cabinetry, assistant position, and patient chair travel determine whether you can bring the optics to the patient—without bringing your neck to the optics.

Quick comparison: where extenders/adapters typically help most

Operatory problem

What you see clinically

Adapter/extender impact

Insufficient microscope reach

You lean forward or rotate to “get under” the scope

Adds working reach so the microscope comes to the field (not your spine)

Awkward viewing angles

Forward head tilt, neck flexion, elevated shoulders

Helps align the optical path with a more neutral head/torso position

Compatibility between components

Delays, “workarounds,” less consistent positioning

Creates a stable, repeatable setup across manufacturers and accessories

Team positioning conflicts (assistant or hygiene)

Bumping arms/booms, constant repositioning mid-procedure

Improves clearance and workflow so you reposition less (and stay neutral more)

Clinical reminder: Even small sustained trunk or neck inclines can drive muscle fatigue over time—one reason “neutral posture” matters more than most people think. (pmc.ncbi.nlm.nih.gov)

Step-by-step: how to evaluate your microscope ergonomics in 15 minutes

1) Start with a “neutral posture checkpoint”

Before touching the microscope: sit/stand in your ideal working position—ears over shoulders, shoulders relaxed, elbows near your torso, wrists neutral. If your microscope forces you out of this position to see clearly, that’s a configuration issue—not a “you problem.”

2) Confirm patient positioning is doing the heavy lifting

Many posture breakdowns come from “patient too high/low” or “head not rotated/tilted enough.” Aim to position the patient so you can keep your spine neutral while the microscope aligns to the field. If you’re consistently craning forward, your operatory routine needs a reset.

3) Watch for the three red flags that indicate you need an extender

• You “run out of travel” and can’t get the head where you need it without leaning.

• You frequently loosen/tighten joints because the ideal position is just beyond reach.

• You can get the view, but only with shrugged shoulders or a rotated trunk.

4) Check repeatability: can you re-create your best setup quickly?

The best ergonomic setup is the one you can reproduce between patients. If every case requires a “microscope wrestling match,” consider whether an adapter improves compatibility or whether an extender improves reach and clearance so positioning becomes routine.

5) Add microbreaks and stretching—because even perfect posture has limits

Neutral posture reduces strain, but static posture (even “good” static posture) still accumulates fatigue. The American Dental Association emphasizes practical ergonomics habits like stretching and microbreaks as part of musculoskeletal health. (ada.org)

Microscope vs loupes: an ergonomic perspective (without the hype)

Loupes are popular because they’re accessible and relatively easy to adopt, and they can support posture improvements when fitted correctly. However, literature and professional discussions commonly point out limitations like fixed magnification ranges and head-position sensitivity, while microscopes offer more adjustability and can reduce postural deviation when properly set up. (pmc.ncbi.nlm.nih.gov)

The most practical framing for many practices isn’t “either/or,” but “match the tool to the procedure and your body.” If your neck and shoulder load is creeping up, the best next step is often a workflow and setup assessment—then decide whether the fix is positioning, equipment configuration, or an accessory (adapter/extender) that makes neutral posture achievable.

Did you know? Quick facts worth sharing with your team

• A systematic review/meta-analysis reported a pooled MSD prevalence of 78.4% among dental healthcare providers. (pmc.ncbi.nlm.nih.gov)

• Studies discussing visual aids note that microscopes differ from loupes in a key ergonomic way: the microscope is not worn and is highly adjustable for a more erect working posture. (pmc.ncbi.nlm.nih.gov)

• Ergonomic programs aren’t just equipment—routine stretching and microbreaks are frequently recommended for preserving musculoskeletal health in dentistry. (ada.org)

A United States perspective: consistency across multi-location teams

For DSOs, multi-doctor practices, and providers who rotate between operatories, ergonomic consistency is a real operational issue. Standardizing microscope positioning habits—and using adapters/extenders to make setups more compatible and repeatable—can reduce “relearning” an operatory each day. That consistency also helps with onboarding associates and supporting long-term clinician wellness.

A simple standardization tip

Create an “ideal setup checklist” for each operatory (chair height range, typical patient head position by quadrant, microscope head position landmarks). Then evaluate whether your hardware makes that checklist achievable without strain—if not, an extender or adapter is often the most efficient path to repeatability.

Need help matching adapters/extenders to your microscope and operatory layout?

DEC Medical has supported medical and dental teams for decades with surgical microscope systems and high-quality adapters and extenders designed to improve ergonomics, reach, and cross-compatibility. If you want a second set of eyes on your setup, the fastest path is a short configuration conversation.

Contact DEC Medical View Products & Microscope Accessories

FAQ

Do dental microscopes really help with neck and back strain?

They can—especially because microscopes are adjustable and not worn on the head. But the benefit depends on correct positioning and a layout that lets the scope reach the field without you leaning. (pmc.ncbi.nlm.nih.gov)

What’s the difference between a microscope adapter and an extender?

In practical terms, an adapter helps components interface correctly (mounts, accessories, compatibility between systems). An extender helps with reach/clearance and positioning, so the microscope can be placed where you need it while you maintain neutral posture.

I have loupes—should I switch to a dental operating microscope?

Not always. Loupes can support ergonomic improvements when properly fitted, and they’re excellent for certain workflows. A microscope can add adjustability and lighting/visual advantages, but it’s best evaluated based on your procedures, operatory constraints, and whether your posture can stay neutral day after day. (pmc.ncbi.nlm.nih.gov)

How do I know if I need an extender?

If you’re repeatedly leaning forward, rotating your torso, or “running out of travel” when positioning the microscope head—those are common signs that reach/clearance is limiting neutral posture and workflow repeatability.

What else should we do besides equipment changes?

Build short microbreaks into your schedule, use simple stretching routines, and train the whole team on consistent patient positioning. Ergonomics is most effective when it’s practiced daily, not only purchased. (ada.org)

Glossary

DOM (Dental Operating Microscope)

A magnification and illumination system mounted to a stand/arm that provides adjustable magnification and a stable, well-lit view of the operating field.

MSD (Musculoskeletal Disorder)

Pain or injury affecting muscles, joints, tendons, or nerves—often associated with repetitive movement and prolonged static posture in clinical work. (pmc.ncbi.nlm.nih.gov)